Mobile charge induced periodic poling and device

ABSTRACT

Devices and methods are disclosed for realizing a high quality bulk domain grating structure utilizing mobile charges that are generated by means of photo-excitation in a substrate. An effect of light exposure (UV, visible, or a combination of wavelengths) is to generate photo-induced charges. The application of a voltage across the substrate combined with the application of light exposure causes a photo-induced current to flow through the substrate. The photo-induced charges (behaving like virtual electrode inside the material) and the photo-induced current result in both reduction of the coercive field required for domain inversion in the material and improve realization of the domain inversion pattern, which previously has not been possible at room temperature.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S. Ser. No. 11/530,336,filed Sep. 8, 2006, now U.S. Pat. No. 7,436,579, which application isfully incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to high quality short period bulkdomain inversion structures (gratings), and more particularly to highquality short period bulk domain inversion structures (gratings) thatare fabricated in substrate materials such as MgO doped congruentlithium niobate using electric field poling.

2. Description of the Related Art

Quasi phase matching (QPM) is an efficient way to achieve nonlinearoptical interactions. The approach was first proposed by Bloembergen etal. (U.S. Pat. No. 3,384,433), using a domain inversion gratingstructure to achieve QPM. Such a domain grating structure can beusefully realized in an optically transparent ferroelectric material,such as lithium niobate (LiNbO₃), lithium tantalate (LiTaO₃) andpotassium titanyl phosphate compounds (KTP). There are many differentways to achieve inverted domain structures in these materials.

A periodic poled material structure can be grown directly within thematerial by modifying a parameter during the growth process, such astemperature, or a dopant concentration. Ming et al. (“The growthstriation and ferroelectric domain structures in Czochralski grown LiNbO₃ single crystals,” Journal of materials Science, v11, p. 1663, 1982.)used variation of temperature, growth rate and solute concentrationduring Czochralski growth to create a periodic structure in LithiumNiobate. Laser heated pedestal growth is disclosed in U.S. Pat. No.5,171,400 by Magel et al. from Stanford University. This method canproduce gratings with periods as short as 6 μm and 4 μm, but it isdifficult to grow long lengths and curvature of the domains limits thelateral dimensions and efficiency.

Impurity doping or material removal in some ferroelectric materials(such as lithium niobate and KTP) can result in domain inversion. Inlithium niobate, periodic domain inversion gratings can be achievedthrough high temperature processes such as titanium indiffusion, lithiumoutdiffusion (in air, or enhanced with surface layers of SiO2 and MgO)or proton exchange. A mechanism for the domain inversion was proposed byone of the present inventors, based on space charge field of impuritygradients (Huang et al. “A discussion on domain inversion in LiNbO ₃ ”,Appl. Phys. Lett. v65. p. 1763, 1994). Byer et al. at StanfordUniversity (U.S. Pat. No. 5,036,220) demonstrated a waveguide frequencyconverter wherein the domain structure was created using titaniumindiffusion in lithium niobate.

Due to the typically shallow impurity diffusion depths, the inverteddomains are also typically shallow and generally triangular orsemicircular in depth in lithium niobate.

A high voltage may be used to generate domain inversion at roomtemperature. Papuchon (U.S. Pat. No. 4,236,785) demonstrated patternedelectric field in-plane poling on lithium niobate to achieve waveguidequasi-phasematched nonlinear interactions. Short period domain inversionin Z-cut congruent lithium niobate (CLN) was first demonstrated byYamada at Sony in 1992 (U.S. Pat. No. 5,193,023) but the describedprocess suffered from limitations in the material thickness and highinstances of destructive electrical breakdown. Since this first reportmany different techniques of applying the electric field have beendemonstrated, generally enabling electric field induced domain inversionto be achieved at or near to room temperature, in contrast to themethods of impurity diffusion. Approaches include the use of patternedmetal electrodes, patterned insulators with liquid electrodes (e.g.,U.S. Pat. No. 5,800,767 and U.S. Pat. No. 5,519,802) and coronadischarge (Harada et al. “Bulk periodically poled MgO:LiNbO ₃ by coronadischarge method”, Appl. Phys. Lett V 69, #18, p2629, 1996, Fuji PhotoFilm Co Ltd). The common feature of all of these approaches is thecreation of a localized electric field modulation (or patterned electricfield) on one face of the substrate.

Bombardment with a high energy electron beam can be used to induce bulkdomain inversion in congruent lithium niobate at room temperature asdemonstrated by Yamada at Sony (Yamada et al. “Fabrication ofperiodically reversed domain structure for SHG in LiNbO ₃ by directelectron beam lithography at room temperature,” Elect. Lett. Vol 27 p.828, 1991), without the use of an applied voltage. Ito et al. alsoperformed electron beam writing of domain gratings in lithium niobate(Ito et al., “Fabrication of periodic domain grating in LiNbO3 byelectron beam writing for application of nonlinear optical processes”Elect. Lett. Vol 27 p. 1221, 1991). The high energy electrons incidenton the substrate penetrate the surface and are trapped inside thesubstrate. These localized trapped electrons in the material result inlocalized high electric field that causes domain inversion. Earlier workby Keys et al. (“Fabrication of domain reversed gratings for SHG inlithium niobate by electron beam bombardment”, Electronics Letters, V26,#3 p 188, 1990) used a mask to pattern the bombardment of a high energyelectron beam on congruent lithium niobate and, combined with anelevated temperature and a small applied voltage, provided patterneddomain inversion.

In essence, all the methods described above are electric field poling.The orientation of the internal dipole moment is reversed under theinfluence of the local and global electric field. In direct growth, andimpurity diffusion approaches the electric field is generated from atemperature gradient, or a dopant gradient. With electron beambombardment the electric field is created by the trapped electronsinjected into the substrate from a high energy beam.

Early work in electric field poling for QPM applications concentratedlargely on congruent lithium niobate since this is by far the mostwidely available nonlinear optical material and also one of the mostversatile, with a transparency range from about 400 nanometers (nm) to 5microns (μm) in wavelength. However, as applications have come to bedeveloped for the visible spectrum, the large numbers of defects in thecongruent crystal structure, together with trace impurities incorporatedduring the growth process, give rise to a property calledphotorefractivity. The photorefractive effect is caused by thedirectional drift of photo-excited charges generated by absorption ofvisible and ultraviolet (UV) light within the material, which creates aspace-charge electric field. The space-charge electric field leads, viathe electro-optic effect, to a refractive index change which distortsthe optical beam passing through the crystal. In order to be used inapplications using or generating visible light, congruent lithiumniobate needs to be doped with about 5% MgO, as shown by Bryan et al.(“Increased optical damage resistance in Lithium Niobate,” Appl. Phys.Leff. V44. p 84, 1984) to overcome the effects of structural defects andeliminate the photorefractive effect.

However the MgO dopant in MgO:CLN brings an even bigger challenge inrealizing periodic domain structures. Many groups of researchers aroundthe world have been working on electric field poling of MgO:CLN. Forexample, corona poling was attempted by Fuji (Harada et al “Bulkperiodically poled MgO—LiNbO3 by corona discharge method” Appl. Phys.Lett. V69 p 2629); the use of elevated temperatures was attempted byMitsubishi Cable (U.S. Pat. No. 6,565,648), and Matsushita (Mizuuchi etal. “Electric field poling in Mg doped LiNbO ₃ ”, Jnl Appl Phys, V96,#11, 2004, Mizuuchi et al “Efficient second harmonic generation of 340nm light in a 1.4 μm periodically poled bulk MgO:LiNbO ₃ ”, Jpn J ApplPhys V42, p 90-91, 2003); ultra-violet light and laser light energyassisted poling has been attempted by several other groups (Muller et at“Influence of ultraviolet illumination on the poling characteristics oflithium niobate crystals” Apl Phys Lett V83 #9 p 1824 2003, Valdivia etal “Nano scale surface domain formation on the +Z face of lithiumniobate by pulsed ultraviolet laser illumination,” Appl Phys Lett V862005, Fujimura et al. “Fabrication of domain inverted gratings inMgO:LiNbO ₃ by applying voltage under ultraviolet irradiation throughphotomask at room temperature”, Elect Lett V39 #9 p 719 2003, Dierolf etal “Direct write method for domain inversion patterns in liNbO3”, AplPhys Lett V84 #20 p 3987 2004). However, short-period-domain-gratingstructures have not been achieved at room temperature in a reliable andrepeatable manner.

Part of the difficulty in poling MgO:CLN is the observation that thereis current flow through the substrate other than the poling displacementcurrent during the poling process. This current flow results inpreferential growth of domains which are formed early in the polingprocess and disrupts the domain seeding uniformity and therefore theuniformity of the final grating pattern.

It is also found that the domain wall boundary in Mg doped CLN seems tobe aligned less rigidly along the crystal axis than in the undoped CLNmaterial. Since the inverted domain structure does not strictly followthe crystal structure, it is fundamentally challenging for the inverteddomain to propagate through the entire thickness of the substrate whilemaintaining the lateral dimensions of the masking pattern applied on onesurface of the substrate.

Accordingly, there is a need to provide an improved domain invertedgrating device with high efficiency and high resistance tophotorefractive effects and a fabrication method able to control thedomain growth through the bulk of the crystal for short period domaininversion gratings for applications in high power visible lightgeneration.

SUMMARY

Accordingly, an object of the present invention is to provide a domaingrating device, and its associated fabrication methods, that hascontrolled domain growth through the bulk of a crystal substrate foruniform short period domain inversion gratings.

Another object of the present invention is to provide a domain gratingdevice fabrication method, using the generation of mobile charges in acrystal substrate to improve the seeding of inverted domains and toguide the growth of the domains through the bulk of the crystalsubstrate for improved poling quality.

Another object of the present invention is to provide an improved domaingrating device that results in high efficiency bulk domain gratingdevices for applications in generating high power visible laser light.This is achieved in a domain grating device that has a crystal substratewith first and second opposing surfaces. The substrate has an inverteddomain grating structure that extends through the entire substrate. Aninverted domain average duty cycle at the first surface is greater than50% and less than 100%, and an inverted domain average duty cycle at thesecond surface is less than 50% and greater than 0% ensuring a region of50% duty cycle within the substrate.

In another embodiment of the present invention, a method is provided forcreating an improved domain grating device. A crystal substrate isprovided with first and second opposing surfaces. Optical illuminationis used to generate mobile charges and patterned current flows. Aninverted domain grating structure is formed that extends through theentire substrate. An inverted domain average duty cycle at the firstsurface is greater than 50% and less than 100%, and an inverted domainaverage duty cycle at the second surface is less than 50% an greaterthan 0%.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a prior art domain grating device.

FIG. 2 illustrates one embodiment of a domain grating device with atapered domain grating of the present invention.

FIG. 3 illustrates a calculation of device conversion efficiency versusgrating duty cycle.

FIG. 4 illustrates a calculation of device conversion efficiency as afunction of depth for one embodiment of the present invention.

FIG. 5 illustrates an optical beam position for optimum performance forone embodiment of a domain grating device of the present invention.

FIG. 6 illustrates an electrode configuration with uniform illuminationthrough the back face for one embodiment of the present invention.

FIGS. 7 a and 7 b illustrate the effect of domain inversion maskmisalignment on poling for one embodiment of the present invention.

FIGS. 7 c and 7 d illustrate the effect of 3-dimensional tapering on theshape of the inverted domain.

FIG. 8 illustrates a typical experimental configuration for anIlluminated electric field poling process that can be used with oneembodiment of the present invention.

FIG. 9 illustrates characteristic electrical traces for a single pulseIlluminated poling process for one embodiment of the present invention.

FIG. 10 illustrates characteristic electrical traces for a dual pulseIlluminated poling process for one embodiment of the present invention.

FIG. 11 illustrates an insulating mask electrode configuration withillumination through the back face for one embodiment of the presentinvention.

FIG. 12 illustrates an Insulating mask electrode configuration with UVillumination through the front face for one embodiment of the presentinvention.

FIG. 13 illustrates interference UV pattern poling for one embodiment ofthe present invention.

FIG. 14 illustrates the combination of a UV interference pattern with aninsulating mask layer for poling in one embodiment of the presentinvention.

FIG. 15 illustrates E-beam induced poling for one embodiment of thepresent invention.

FIG. 16 illustrates a single pass frequency doubled visible laser sourceusing a domain grating device for one embodiment of the presentinvention.

FIG. 17 illustrates an Intra-cavity frequency doubled visible lasersource using a domain grating device for one embodiment of the presentinvention.

FIG. 18 illustrates a projection laser light display system for oneembodiment of the present invention.

FIG. 19 illustrates a scanning laser light display system for oneembodiment of the present invention.

FIG. 20 illustrates a difference frequency generator using a domaingrating device for one embodiment of the present invention.

DETAILED DESCRIPTION

In one embodiment of the present invention, an improved domain inversionstructure is provided that has optimized efficiency, reliablefabrication and ease of characterization. This improved domain inversionstructure is the result of a new high voltage electric field polingbased fabrication process which involves the generation of mobilecharges within the substrate that is to be poled, accompanied by theapplication of a patterned high voltage electric field. This combinationresults in a patterned current flow through the substrate and creates apatterned domain inversion structure within the substrate. Variouscombinations of charge generation and voltage application can be used totailor the size and shape of the domain inverted regions.

In one embodiment, as illustrated in FIG. 2, a frequency conversiondevice 200 is provided. An improved domain inversion structure resultingfrom a new fabrication process is shown schematically in FIG. 2. Theinverted domains 202 are tapered in size from one face 204 of a crystalsubstrate 201 (also referred to herein as “the crystal” and “thesubstrate”) to the other face 205, and may extend through the entirethickness 206 of the substrate 201. Domain inverted devices in lithiumniobate made by the prior art fabrication techniques have been unable toprovide this combination of advantageous properties. A typical prior artdevice is shown in FIG. 1. Here, although the domain walls extendthrough the entire thickness 106 of the substrate 101, they aresubstantially parallel to the Z-axis and the dimensions of the inverteddomains 105 are substantially constant through the material thickness.While, in principle, this constant domain dimension could be seen as agood property since it allows the fabrication of high aspect ratiodomains, and provides a uniform grating structure throughout thethickness of the substrate, it is also exceedingly difficult tofabricate the grating so that the domains have exactly the optimumdimensions.

In frequency conversion applications utilizing a first order QPM gratingthe optimum efficiency is achieved with a 50/50 duty cycle between thetwo anti-parallel domain orientations. The importance of the domain dutycycle can be seen in FIG. 3, where the efficiency of a phase-matchedinteraction is plotted as a function of the duty cycle of the grating.

When the poling process is controlled by computer based on the amount ofcharge that has been transferred onto the substrate (a measure of theamount of domain inversion that has occurred), if there are a number ofdefects in the lithographically patterned poling mask that cause anumber of domains to merge together, the charge involved in causing themerging will result in less domain inversion in the remaining pattern,potentially resulting in a lower than desired duty cycle overall. Sincein undoped congruent lithium niobate the inverted domain is generallyuniform in dimension throughout the bulk of the substrate, a domaingrating that is either over or under duty cycle at the surface as aresult of one of the above parameters will not provide optimumconversion efficiency. FIG. 1 depicts the conventional grating device101 from prior art process described in reference (U.S. Pat. No.5,800,767). In this device, the domain grating is over duty cycle (i.e.,the inverted domain 105 is larger than the un-inverted domain 104 in thegrating region). Once the device is fabricated, the efficiency of thedevice is fixed and any difference in duty cycle from the optimum 50/50results in a decrease in performance compared to the optimum device.

In one embodiment of the present invention, a tapered domain gratingdevice is provided where an ideal first order QPM 50/50 duty cycle isrealized inside the substrate for efficient nonlinear interactions asshown schematically in FIG. 2. The inverted domains 202 are tapered insize from one face of the substrate to the other and may extend throughthe entire thickness of the substrate 206. Even if the domain inversionis not precisely controlled at the patterned surface, the taper of thedomains in the present invention provides for a 50/50 duty cycle regionwithin the bulk of the material where the dimensions of the inverted 202and uninverted 203 domains are matched. For example, for a gratingperiod of 4 μm, a substrate thickness of 0.5 mm, an inverted domain 202dimension of 3 μm on the top (patterned) face and 1 μm on the bottom,there will be an approximately 120 μm thick region within the substratewhere the duty cycle ranges from approximately 47% to approximately 53%,for which the efficiency is 98% of the theoretical maximum, as indicatedin FIG. 4. Even when a more strongly tapered domain merges on the frontface, FIG. 4 illustrates that as long as the domain is under duty cycleon the back face (less than 50/50), there still a region inside thematerial where the 50/50 duty cycle is found. Thus, an advantage of theimproved domain inversion structure of the present invention is thateven if the duty cycle of the domain inversion structure is notprecisely controlled to be 50/50 at the patterned surface, the taper ofthe domains provides for a 50/50 duty cycle region within the bulk ofthe material where the dimensions of the inverted 202 and uninverted 203domains are matched. It is well understood in the art that random domainsize variations occur as a result of random delay times for theinitiation of domain inversion after the application of the polingvoltage such that even a uniform grating is understood to have somesmall variation in domain feature size and duty cycle along and acrossthe extents of the grating area. In practice, it is noted that therandom domain size variations which occur during the fabrication processmay result in some domains being smaller than 50% duty cycle on thepatterned face, while some other domains are greater than 50% duty cycleon the unpatterned face. This variation causes a slight decrease ineffective conversion efficiency for the structure. However, as long asthe average domain duty cycle on the patterned face is greater than 50%and the average duty cycle on the bottom face is less than 50%, theefficiency optimization described above can be performed.

When the device of FIG. 2 is used as a frequency conversion element,adjustment of the optical beam position with respect to the depth of thesubstrate will result in an optimally high efficiency nonlinearinteraction as indicated in FIG. 5, wherein the beam 502 is at theposition having the optimum efficiency (whereas the beams 501 and 503are at positions of sub-optimum efficiency corresponding to non-50/50duty cycles). This post-fabrication efficiency optimization is notpossible for devices produced from prior art processes with verticaldomain walls. Since the exact duty cycle of the domain grating on themasked face of the device of FIG. 2 is not critical as long as it isgreater than 50/50 and the domain taper angle is controlled such thatthe duty cycle on the unmasked face is less than 50/50, the degree ofcontrol required over the poling process is relaxed compared to thatrequired to fabricate an exact 50/50 duty cycle grating with verticalwalls. Thus, the tapered domain structure greatly increases the polingprocess latitude in terms of exact current and charge control in a massproduction process, while preserving the ability to provide optimumdevice efficiency. The trade-off in this case is that the optimum dutycycle region of the substrate is necessarily limited by the domain taperangle. Therefore the conversion efficiency uniformity of the device ofFIG. 2 in the vertical direction through the substrate will generally beworse than that of the device of FIG. 1 with vertical domain walls, butthe peak conversion efficiency will generally be higher.

Highly efficient domain gratings can be achieved utilizing this devicestructure. For example, an effective nonlinear coefficient of greaterthan 16 picometers/volt (pm/V) has been achieved for blue frequencydoubling with a grating period of 4.45 μm.

In one embodiment of the present invention, fabrication methods areprovided that involve the generation of mobile charges within thesubstrate that is to be poled, accompanied by the application of apatterned high voltage electric field. This combination results in apatterned current flow through the substrate and creates a patterneddomain inversion structure within the substrate. Various combinations ofcharge generation, voltage application and patterning mask can be usedto tailor the size and shape of the domain inverted regions.

One basic approach of the invention is shown in FIG. 6 and combines ahigh voltage electric field applied between a plurality of lines 602forming a patterned electrode on one face 603 of a substrate 601 and auniform transparent electrode 605 on the other face 610, and atransversely spatially uniform light illumination 606 incident to thesubstrate through the transparent electrode. The transversely spatiallyuniform light illumination creates photo-induced charges within the bulkof the substrate with a density and distribution dependent on theillumination intensity, illumination wavelength(s) and the absorption ofthe substrate. The applied high voltage electric field causes aphoto-current to flow through the substrate due to the presence of thephoto-excited charges. The patterned nature of the electric field (or inother terms, the patterned nature of the photocurrent flow on one faceof the substrate arising from the patterned electrode) and the mobilecharge depth profile in the substrate resulting from substrateabsorption, define the pattern of the resulting domain invertedfeatures. The patterned electrode in FIG. 6 is depicted as a patternedmetal layer including a plurality of lines 602, but it may also take theform of a patterned insulating layer overlaid with a conductor such as ametal or a liquid as described in greater detail below. In this case,the patterned insulating layer preferably acts as a current mask topattern the flow of the photocurrent in response to the illumination andthe applied field.

the patterned insulating layer may also provide a modulation of theelectric field strength within the substrate. The mobile charges whichare generated in the bulk of the substrate by the spatially uniformillumination, and which form the photocurrent in response to the appliedelectric field, are constrained to flow through the patterned maskapertures, creating a large current density at the apertures whichprovides a strong effect in lowering the coercive field (here thecoercive field is macroscopically defined as the applied voltage thatinduces polarization reversal in the substrate, divided by the thicknessof the substrate) and enabling the controlled seeding of patterneddomain inversion. The present inventors have observed that the effectivecoercive field of the substrate is generally decreased as the magnitudeof the photocurrent is increased, for instance by increasing theintensity of the illumination, or by altering the spectrum of theillumination.

Here, transversely spatially uniform light illumination means that theincident illumination is substantially uniform in intensity and spectrumacross the transverse dimensions of the illuminated portion of thesubstrate surface, which is generally equivalent to the poling area.This uniformity is highly desirable in order to achieve a uniformspatial domain inversion pattern over the entire poling area. It shouldbe noted that the illumination will not be uniform in the direction ofthe light propagation in the bulk of the material since absorption willdecrease the intensity and change the spectral mix of the illuminationas the light propagates into the material. The transverse uniformity ofthe domain inversion pattern should be maintained as long as theproperties of the bulk substrate are uniform over the transversedimensions of the illuminated region. The phrase “uniform illumination”should be taken to refer to illumination which is substantially uniformover the transverse dimensions of the illuminated region. It should alsobe noted that the spatially uniform light illumination does not have tobe spatially uniform at all times. That is, the time averaged intensityand/or spectral content over a time period considerably shorter than thetotal illumination time should be substantially uniform, but theinstantaneous intensity may vary across the illumination area. Therequirement for illumination uniformity is based on a requirement forsubstantially uniform mobile charge generation across the illuminatedregion so that the electric field poling process is initiated atsubstantially the same point in time across the entire aperture, and isable to proceed at substantially the same rate over the illuminatedarea. It is well understood in the art that random domain sizevariations occur as a result of random delay times for the initiation ofdomain inversion after the application of the poling voltage such thateven a uniform grating is understood to have some small variation indomain feature size and duty cycle along and across the extents of thegrating area. In the presence of a material non-uniformity, such as adopant concentration non-uniformity, the illumination uniformity can betailored to result in uniform mobile charge generation in the substrate.That is, the illumination intensity can be varied transversely acrossthe substrate such that even in the presence of the dopant concentrationnon-uniformity, the mobile charge generation remains substantiallyuniform and the resulting domain inversion pattern is also substantiallyuniform. The applied electric field should also preferably besubstantially uniformly applied across the illuminated area in order toprovide substantially uniform domain inversion (i.e., there should be nosignificant monotonic field variation across the transverse dimension ofthe substrate), although localized modulation of the electric field isdesired for optimum poling. In the case of the material non-uniformitydescribed above, the strength of the applied electric field could bevaried across the transverse dimensions of the poling area instead ofthe illumination uniformity, in order to counteract the materialuniformity. Alternatively, both the electric field and the illuminationmay vary across the poling area in such a way that their combinationproduces a uniform poling pattern, with or without the presence of amaterial non-uniformity.

The distributed nature of the photo-charges within the material and thepatterned photo-current flow effectively define the domain inversionpattern throughout the bulk of the substrate, leading to very highaspect ratio domain inversion features and very high quality shortperiod domain inversion gratings. The current masking windows in thepatterned electrode on one of the surfaces of the substrate, and thehigher conductivity of the seeded domain walls growing in the bulk ofthe substrate enable a patterned current flow through the bulk of thesubstrate due to the photo-excited charges. These “patterned” movingcharges can be envisioned as a virtual electrode in the bulk of thesubstrate.

Controlling the combination of applied voltage, illumination intensity,illumination wavelength(s), illumination time, photocurrent/charge andpoling charge, enables control to be exercised over the size and shapeof the resulting domain inverted features. It has been shown by thepresent inventors that with the appropriate illumination and voltageparameters (as described in the following preferred embodiments) it ispossible to create a substantially uniform short period (<4 micron)domain inversion grating in even 1 millimeter (mm) thick MgO:CLNsubstrates. The domain inverted features can propagate through theentire thickness of the substrate and can be observed by HF(hydro-fluoric acid) etching on both the front and back faces of thesubstrate. Thus, domains with aspect ratios of greater than 250:1 can befabricated substantially uniformly over large areas limited only by theuniformity of the optical illumination, electric field application, andthe uniformity of the substrate material itself.

Highly efficient frequency conversion devices can be fabricated usingthe new fabrication process described here. For example an effectivenonlinear coefficient of greater than approximately 16 picometer/volt(pm/V) has been achieved for blue frequency doubling with a gratingperiod of 4.45 μm.

In one embodiment of the present invention, the combination of patternedelectric field and uniform illumination minimizes merging and providesimproved uniformity and repeatability of domain inversion as compared toa patterned electric field alone. In addition, the peak high voltagethat must be applied in conjunction with the uniform illumination issignificantly lower that that required for uniform domain seeding atshort periods without illumination, which substantially eliminates thepossibility of destructive electrical breakdown of the substrate duringpoling. This is especially important when considering scaling theelectric field poling process to full wafer areas, and enables a farmore robust process for high yield, high volume manufacturing than thatof the application of a high voltage alone.

It has also been determined by the present inventors that theilluminated poling process of the present invention is more resistant tothe deleterious effects of thermally induced domain inversion defects.These thermally induced domain inversion defects, often called “heatdefects”, result from the lithographic processing of Z-cut lithiumniobate wafers. The heating and cooling cycles during the lithographyprocess lead to the buildup of pyroelectrically generated charges on thewafer surface, which can lead to the spontaneous domain inversion ofsmall defect-seeded regions. Thus, a wafer that was uni-domain at thestart of the lithographic process may end up with a large number ofsmall, isolated domain inverted regions within it by the end of theprocess. During the electric field poling process these domains causemerging of the desired pattern and tend to grow to the detriment of thedesired domain pattern, leading to reduced domain inversion patternquality. The present illuminated poling invention has been found toresist the effects of these heat defects and to suppress the tendencyfor the formation of large merged regions around each defect site duringthe poling process. Thus the desired domain inversion pattern can stillbe uniformly seeded and grown with good pattern quality, even when thepoled area is increased to allow for wafer scale processing for highvolume manufacturing.

It should be noted that others have attempted to use the formation ofsuch “heat defects” as a method of seeding the domain inversion processin MgO:CLN to create uniform domain inversion gratings (Nakamura et al.,“Periodic poling of magnesium oxide doped lithium niobate”, Jnl. Appl,Phys, Vol 91, No 7, 2002, p 4528-34). However, the random nature of the“heat defect” locations makes this method impractical for short periodgratings. and with the application of a high voltage pulse alone, thepreferential expansion of the “heat defects” during the poling pulsecauses significant merging and loss of quality in the domain grating.

The size and shape of the inverted domain feature can be controlled bysetting appropriate values for the illumination and voltage applied tothe substrate. In particular it has been found that varying theillumination spectrum can be used to control the degree of penetrationof the inverted domain into and through the substrate for given set ofpoling parameters. With the appropriate illumination spectrum, whichcontains some optical power at wavelengths below about 320 nm, it ispossible to terminate the domains before the illuminated face (theunpatterned face) of the substrate. If the wavelengths of light belowabout 320 nm are removed from the illumination spectrum, using anabsorption filter for example, full penetration of the domains throughthe substrate can be readily achieved. Even with these wavelengthsremoved from the spectrum, it is still possible to terminate the domainswithin the bulk of the substrate by decreasing the illumination timeand/or the illumination intensity compared to the values used to producea fully penetrated domain inversion pattern.

In another embodiment of the present invention, domain inversion of 0.5mm thick ˜5% MgO doped congruent lithium niobate is achieved using apatterned metal electrode on one face and a transparent planar electrodeon the other face, combined with UV/visible illumination through thetransparent electrode. Referring again to FIG. 6, a patterned metalelectrode 602 is disposed on a first face (generally termed the “front”face) 603 of the substrate 601, preferably the +z face, using standardphotolithographic techniques as follows. Firstly, the wafer surface ischemically cleaned using buffered oxide etch (BOE), acetone andisopropyl alcohol (IPA) in sequence. An oxygen plasma ashing process isused to ensure the removal of any remaining hydrocarbon contamination. Ametal layer, for example tantalum, is deposited on the cleaned substratesurface using a sputter or e-beam deposition process. Care is taken tochoose the suitable metal to use on the suitable surface of a givenferroelectric material. A layer of photoresist (e.g., Shipley 3312 or AZ5214) is spun onto the metal layer and soft-baked to remove the excesssolvent. The photoresist is exposed using standard lithographictechniques (contact or projection lithography) and developed into thepattern of the desired electrode. The metal is than etched either usingwet chemical etching or plasma etching to form the patterned electrode.Alternatively the patterned electrode fabrication can be performed usinga lift-off process where the photoresist is patterned on the substratefirst, and then metal is then deposited using an e-beam depositiontechnique, for example. The metal that is deposited over the top of theresist is removed (or “lifted off”) using solvent typically withultrasonic agitation, while the metal deposited on the substrate surfaceremains. After the metal electrode is patterned, an insulatingdielectric layer 607 is preferably deposited to cover the metalelectrode. This dielectric layer should have good electrical insulatingproperties especially at the contact interface between the dielectriclayer and the substrate surface. While the patterned metal electrodeprovides the electric field modulation and the photo-current paths, theinsulating layer between the lines provides a current masking capabilityto limit the photo-current to flow only to the metal lines. Aphotoresist layer (e.g., Shiply 3312 or AZ 4210) can serve as theinsulating layer after it is cross linked (hard baked) at a suitabletemperature (e.g., 140-200° C.) depending on the type of resist.Alternatively an SiO₂ layer deposited by sputtering, evaporation orspin-on-glass can form the insulator. Electrical contact openings, suchas contact opening 608 to the lines 602 of the metal electrode throughthe insulating layer are also provided by well known lithographyprocesses.

With regard to the photolithographically patterned mask used to definethe domain inversion pattern, it has been found by the present inventorsthat a long line feature is not suitable for poling of short gratingperiods due to the fact that domain structure tends to follow thehexagonal crystal structure of the substrate, where one of the sides ofthe hexagon is oriented along the y-axis of the crystal in LiNbO₃. Whenthe line feature in the poling mask is defined at an angle with respectto the y-axis, the resulting poled domain generally either displays ajagged edge or expands outwards from the mask feature to form anelongated hexagonal domain, as illustrated in FIG. 7 a.

The domain expands sideways away from the patterned mask feature 705until the edges of the domain 706 are aligned with the crystal y-axis704. Thus, the width of the domain is no longer defined by the width ofthe photolithographically defined mask, but is defined by the effectivewidth 702 of the lithographic feature perpendicular to the y-axis.Hence, the greater the angle 703 of the feature with respect to they-axis, the greater the width the poled domain becomes before the sidesof the domain are parallel to the y-axis.

For optimal poling quality of fine pitch gratings the poling mask shouldbe aligned so that each grating bar, defined by a mask feature 705, isexactly parallel to the y-axis. However, there are a number ofdifficulties involved in this alignment. Firstly, there is typically atolerance of ±0.25 to ±0.5 degrees in the angular accuracy of theorientation flat provided by the wafer manufacturer. Secondly, thebeveling process applied to the edges of the wafer to remove edge chipsand prevent wafer breakage during processing often leads to a slightcurvature of the orientation flat, further reducing the effectiveaccuracy. Thirdly, some angular error will be introduced when the polingmask is physically aligned to the orientation flat due to the resolutionof the mask aligner and the finite length of the orientation flat at theedge of the wafer.

It can be seen from the domain expansion argument above that, for agiven period of grating 712 in FIG. 7 b, there is a maximum allowablemisalignment angle 710 for a particular length of feature 714.Basically, when the effective width 711 of the feature perpendicular tothe y-axis becomes equal to or greater than half the grating period 712,there is a significantly increased likelihood that adjacent domains willmerge together and the grating structure will be lost.

Thus, for a robust production domain inversion process we can define amaximum line length in the poling mask, such that the achievable angularalignment accuracy does not cause the domain grating pattern to merge.For example, if the alignment accuracy is ±0.5 degrees and the gratingperiod is 4 μm, the maximum feature length that can be allowed is L=2μm/sin(0.5 degrees)=˜230 μm. In this case, a number of line features arepatterned on the mask separated by a small distance to make up the fullwidth of the desired poling region.

It is found that the domain shape resulting from the above describedpoling mask design and the poling process is trapezoidal, and isgenerally tapered in three dimensions, due to domain termination insidethe bulk of the substrate. Besides the effective duty cycle change asdescribed above, the tapering shortens the domain length (poled linelength) along the y-axis on the back, illuminated face compared to thefront, patterned face. This is not generally favorable for the nonlinearoptical frequency conversion interaction, as a reduction in the lengthof the poled line decreases the effective overlap between the opticalbeams and the domain inversion grating. Therefore, the poling parametersare preferably tailored to minimize the shortening of the domain lengthas it propagates through the thickness of the crystal.

FIG. 7 c shows a typical inverted domain 720. In general, the domainshape on the front, or patterned surface 722 of the substrate is similarto the shape of the poling mask, although generally somewhat expandedfrom the size of the mask opening due to the fringe effect at the edgesof the electrodes and domain expansion under the photolithographicallydefined mask. On the back face of the crystal 726 (the illuminated andunpatterned face) the domain shape is generally more hexagonal in naturewith points at the + and −y-axis ends of each poling line. The width 723of the inverted domain 720 on the front surface 722 of the substrate iswider than the width 724 or 725 of the inverted domain on the backsurface 726. This illustrates the domain taper in the x-direction withdepth through the substrate, which is used in the present invention toobtain an optimal 50-50 duty cycle grating structure within thesubstrate. The inverted domain width on the bottom face is alsogenerally tapered from larger on one end 724 to smaller on the other end725. The larger end of the domain is generally oriented towards the −yaxis of the crystalline substrate. Additionally, the length of thedomain line is generally longer on the front face 722 than the length onthe back face 726, illustrating that the domain feature also tapers inthe y-direction with depth through the substrate. FIG. 7 d shows a topview of an inverted domain where the domain length 732 on the front,patterned surface is longer than the length 734 on the back, unpatternedsurface. The width 733 of the domain at the top surface is wider thanthe width of the domain 735 and 736 on the back surface, where width 736is larger than width 735.

The shortening of the domain line length on the back face of thesubstrate means that there is a reduction of the domain length in thebulk of the substrate. This effectively decreases the overlap of theinverted domain and the optical mode fields and results in a reducedefficiency for the device. To maintain maximum device performance,optimization of the length of the domain on the back face and the taperof the width of the domain on the back face can be performed byadjusting the poling parameters such as UV dose, UV spectrum electricalpulse waveform and combinations of these parameters. For example, ahigher voltage spike pulse early in the poling process can increase thedomain seeding and growth on the ends of the domain feature, which canmore uniformly define the width of the domain at both ends. The lengthof the spike pulse can be optimized to reduce the domain, lengthreduction by optimizing the growth of the seed domain along the entirelength of the domain grating bar feature. These modifications can beperformed independently or together with the optimization of the dutycycle as described in the embodiments above. Preferably, the length ofthe poling line may be shortened by no more than about 0.5 um throughthe thickness of the substrate (i.e., the length of a domain at thefront face should be within about 0.5 um of the length of the domain onthe back face. The width of the domain on the bottom face towards the +yend of the domain may be smaller than the width at the −y end of thedomain by no more than about 0.5 um, and preferable by even less thanthis. Practically, a domain width variation of between about 0.25 um andabout 0.5 um from end to end of the domain on the back face of thesubstrate appears achievable.

In the case of the patterned metal mask (poling mask) used in thisembodiment of the invention, the mask features can effectively be brokeninto grating bars (poling bars) of the desired length simply bydepositing and patterning an insulating layer on the surface of thesubstrate before the deposition and patterning of the metal mask layer.The patterned insulating layer should consist of a series of linesdisposed substantially perpendicular to the desired grating bars, andspaced apart by the desired bar length. The metal layer may then bedeposited over the top of the insulator and then patterned to providethe grating bars. Where the patterned insulator is interposed betweenthe metal layer and the substrate, the required voltage to achievedomain inversion will be increased, effectively preventing domaininversion from occurring, and hence breaking the patterned metal polingmask into a number of bars of the desired length. In other embodimentsof this invention, described in greater detail below, a patternedinsulating layer may be used to provide the poling mask. In this case,the poling bars are defined simply by the length of the openings in thephotomask that are transferred into the insulating mask and no extraprocessing is required.

To improve the angular alignment accuracy it is possible to providedomain inverted alignment features which more precisely define thecrystal y-axis direction. An initial poling pattern consisting of a fewnarrow bars parallel to the y-axis is aligned to the wafer orientationflat. The pattern is then poled into the crystal and the domains allowedto expand out from the mask pattern so that their edges are parallel tothe y-axis. The poling mask may then be removed and the crystal surfaceetched in HF to reveal the poled features. Preferably, only the areaimmediately surrounding the poled features is exposed to the HF to avoidpossible damage of the surface still to be poled. A second poling maskconsisting of the desired grating pattern is then aligned using thepoled features to define the crystal y-axis, thus achieving improvedaccuracy between the grating lines and the crystal axis. This two stepprocess allows longer individual lines to be poled than are generallypossible with single step alignment to the wafer orientation flat.

During the lithography process, there are many thermal processes such asresist baking. It is preferable to control the thermal ramp rates of thesubstrate/wafer during these baking processes, and also preferable toprovide some form of discharge path for pyrolectrically generatedcharges. MgO:CLN is very prone to the generation of “heat defects,”regions of domain inversion created as a result of pyroelectric chargeaccumulation on the wafer surface during heating and cooling cycles. Ingeneral the “heat defect” domain inversion sites are problematical forthe fabrication of high quality short period domain inversion gratingssince they tend to lead to merges between adjacent domains and defects,reducing the quality of the grating and the efficiency of any QPMoptical frequency conversion process using the grating.

Despite the observation that the illuminated electric field polingprocess of the present invention is significantly more tolerant of, orresistant to, the deleterious effects of “heat defects” than the priorart electric field poling process, it is still preferable to minimizethe number of defects that are formed in order to maximize the qualityof the final domain inversion grating.

In the poling process, the electrical contact to each line 602 of thepatterned metal electrode may be made by a probe contact 610, asillustrated in FIG. 6. A transparent electrode 605 on the back face iscreated, for example, by a solution of lithium chloride in de-ionizedwater. The liquid can be confined using an O-ring 604 with a quartzcover plate 611, or simply with a tape cutout or a silicone gel orgrease barrier.

A typical experimental setup for the poling process described above isshown in FIG. 8. A computer 805 controls a high voltage pulsed powersupply 806 and a UV/visible light source 804. The electrical contacts808 to the crystal 801 are connected to the high voltage power supply,and a light sensor 807 is preferably incorporated into the circuit toprovide an optical monitor for the computer control. Preferably, thecomputer controls the voltage supply, the light source shutter and thetiming sequence. Alternatively some features such as the illuminationtime may be controlled by independent timers and the optical monitorused to provide process sequencing via the computer. The computer canalso preferably monitor the current in the poling circuit, using forexample an optically isolated current monitor 809, to provide control ofthe charge delivered in the various phases of the process.

The light output from the light source 804, which may be coupled via alight-pipe 803, is arranged to provide sufficient illumination intensityand uniformity across the electrical contact area on the back face ofthe substrate (e.g., as defined by the transparent electrode 605).Typical intensities of ˜10 watts per square centimeter (W/cm²) at theoutput of the light-pipe (broadband, all wavelengths from a highpressure mercury bulb) may be used. A beam shaping/expanding system 802(e.g., a lens system) can be used to increase the illumination beamdiameter and/or uniformity on the substrate. Typically about 0.5 W/cm²of total light intensity is incident at the surface the substrate.Higher and lower intensities may be used with appropriate adjustments inillumination time and applied voltage to achieve domain inversion.

A typical sequence of voltage and UV light poling is shown in FIG. 9, asfollows. An initial voltage 901 of about 2500V (˜5000 V/mm) is appliedto the substrate in the absence of illumination, ramping up from zerovolts in about 60 ms. At this time no poling occurs because the voltageis significantly below the coercive field required to achieve domaininversion, and therefore no current flows in the poling circuit. Theshutter of the illumination source is then opened, illuminating theunpatterned face of the wafer with both visible and ultra violetwavelengths 902 (a simple broadband mercury lamp source may be used, ora combination of one or more narrowband light sources may be used toachieve the same effect, as long as the wavelength(s) and intensity(ies)are chosen so as to produce a similar quantity and distribution ofphoto-excited charges within the substrate). After the illuminationbegins, a photo-induced current 904 starts to flow through the substrateunder the influence of the high voltage 901 applied across it. Thiscurrent generally increases gradually over a time frame of 100ths to10ths of seconds, and then tends to reach a plateau. Once thephoto-current has increased to a sufficient value, which is determinedlargely by the area of exposure and the applied voltage, domaininversion occurs within the substrate despite the applied electric fieldbeing nominally below the coercive field. This domain inversion appearsto be seeded from the patterned face and is thought to result at leastin part from the effects of the concentration of the photo-inducedcurrent flow at the ends and edge of the metal electrode. It is thoughtthat domain inversion occurs even though the applied voltage issignificantly below the nominal coercive field of the material at leastin part because the mobile charges generated in the substrate by theillumination decrease the effective coercive field of the illuminatedmaterial.

The observed current flow 903 is now composed of two components: 1) thephotocurrent 904 due to the charges generated by the illumination, and2) the poling current 905, displacement current due to the domaininversion. Typically the photocurrent remains substantially constantafter its initial growth period, whereas the poling current 905typically increases to a maximum and then decreases again as the polingis completed. Thus, the poling process can be controlled by monitoringthe current flow and terminating the applied voltage 901 when either thecurrent 903 or THE transferred charge reaches some predetermined value(which is also dependent on the magnitude of the photo-current).

After poling, the insulating layer 607 over the metal electrode isstripped off and the metal electrode is etched off. The substrate isthen etched in hydro-fluoric acid to reveal the poled pattern. As notedabove, it has been found that in general the inverted domains resultingfrom the above described process are tapered, with a wider line width onthe patterned electrode face 603, and a narrower width on the face 610with the transparent uniform electrode. The width on the patterned faceand the depth into the substrate both generally increase with increasingvoltage, increasing illumination intensity and increasing illuminationtime.

It has been observed with some combinations of illumination spectrum,illumination dose (i.e., light intensity×time) and applied voltage, thatthe inverted domains are terminated inside the bulk of the crystal anddo not generally reach the uniformly illuminated face. As noted above,it has been found that the inclusion of short wavelength UV radiationaround or below the band gap (˜320 nm in MgO:CLN) in the illuminationspectrum has the effect of terminating the domains in the bulk of thecrystal.

In general, it can be desirable that the domains penetrate completelythrough the substrate for optimum device performance and for ease ofdevice characterization, so it is preferable to filter the illuminationto remove the shortest wavelengths. A dichroic or absorptive filter maybe used to provide selectivity in the wavelengths that are removed.

After etching of the front and back faces of the crystal to reveal thedomain inversion patterns, the quality of the domain inversion gratingdevice can be estimated using by measuring the width of the inverteddomain and the width of un-inverted domain to obtain the duty cycle onthe front and back faces, observing the density of any merged or missingpattern sections, and computing the effective nonlinear opticalcoefficient through the depth of the substrate based on these measuredvalues, as shown in the example of FIG. 4. The wafer substratecontaining the domain inversion gratings may be diced to separate theindividual gratings, which may have different periods corresponding todifferent patterns on the photolithographic mask. The end faces of eachdevice may then be optically polished and, preferably, coated withanti-reflection coatings, ready for use as a quasi-phasematchedfrequency conversion device as shown in FIG. 5. The position of theoptical beam within the crystal can be adjusted in depth as describedabove to utilize the optimum 50/50 duty cycle region of the gratingwhich results from the tapered domain structure.

After poling, different portions of the crystal have opposite domainorientations. There is a resulting crystal discontinuity at the boundarybetween the opposite polarity domains. At this boundary, a refractiveindex pattern can be observed using transmission illumination andcrossed polarizers, or a Nomarski microscope. This refractive indexpattern may be the result of uncompensated charges at the boundary,causing a refractive index change via the electro-optic effect, or fromstress at the boundary via the elasto-optic effect. This refractiveindex pattern becomes less pronounced after the sample is exposed to UVor short wavelength visible illumination, thermal annealing or simplyleft at room temperature for some extended period of time.

In order to effectively use the periodically poled (domain inverted)frequency conversion device for the generation of visible light, thediscontinuity of the crystal at the domain wall boundary needs to beaddressed carefully. The boundary and the associated refractive indexchange can act as an extra scattering source, increasing the opticalloss in the device. In addition, new phenomena such as green induced IRabsorption (GRIIRA) and Blue induced IR absorption (BLIIRA) areassociated with this boundary structure and the defects introduced bythe domain boundary.

In one embodiment of the present invention, to alleviate the effects ofthe boundary defect structure on the visible light generation process, ahigh temperature annealing process is used. A discharging closed loop isformed by placing the domain inverted sample between two semi-conductivesilicon wafers which are electrically connected to dissipatepyroelectric charges. The sample stack is then placed into a hightemperature oven or furnace, typically in an ambient air atmosphere,although alternative oxidizing and reducing atmospheres of, forinstance, oxygen and argon respectively may be preferred for someapplications. The temperature of the furnace is raised slowly from roomtemperature up to typically between 500 C and 600 C in about 5 hours.The samples are left at this temperature for a relatively long period,typically around 48 hours, before being cooled down to room temperature.Preferably, the cooling is performed at a rate of a few degreescentigrade per minute, preferably as low as 0.5 C/min. The electricallyshorted high temperature annealing process significantly improves theperformance of the visible frequency conversion device, especially forshort wavelengths in the blue spectrum, by reducing the boundary defectdensity, uncompensated bonds and charges and stresses at the domainboundaries. For short period frequency conversion devices for visibleapplications care must be taken not to significantly reduce the material(i.e., to use an atmosphere containing at least some oxygen). It is alsonecessary to maintain the annealing temperature below the thresholdwhich causes domain boundary motion and domain merging. In MgO:CLN, thisdomain boundary motion is typically observed in short period domaininversion structures at temperatures in excess of about 650 C,indicating that annealing temperatures are preferably below this value.

As noted above, the exposure of the domain inverted sample to UV andvisible radiation appears to reduce the magnitude of the refractiveindex change at the domain wall boundary. Therefore, it may beadvantageous to illuminate the domain inverted sample with UV and orvisible light during the high temperature annealing process. In thisinstance, transparent conducting material is preferably used for thedischarging loop (e.g., Indium Tin Oxide (ITO) coated quartz) to enablesimultaneous illumination and pyroelectric charge dissipation.

In another embodiment of the present invention, the domain inverteddevice is partially coated with a conductive layer. Preferably thislayer provides a conductive path linking the front and back opposingsurfaces of the domain inverted device. The conducting layer may bedeposited before or after annealing and dicing of the domain inverteddevice. If the layer is deposited before dicing, the conductive path maybe completed after dicing by for instance painting the side face of thedevice with conductive silver paint which spills slightly over onto thefront and back faces. The conducting layer enables the dissipation ofthermally excited charges (pyroelectricity), and also enables thedissipation of photocharges that drift to the edge of the substrate,where they are no longer trapped. Thus, the conductive path over partsof at least three faces of the domain inverted device offers theprospect of decreasing the beam distortion and performance limitingeffects of any residual photorefractivity still present in the domaininverted device.

In one embodiment of the present invention, the domain structure,fabricated in MgO:CLN by using a single applied voltage combined withillumination, as described above, generally has a significant taper fromfront (patterned electrode face) to back (uniform illumination andelectrode face) surfaces. The domain features on the uniformillumination/electrode (back) face are generally very narrow. Thus, theoptimum conversion efficiency region, e.g., the section of the substratewhere the effective nonlinear optical coefficient of the poled structureis greater than some defined level (e.g. 90% of the peak value), isrelatively narrow due to the string domain taper. This is illustrated inFIG. 4, where the comparison is made between a substrate with 100% dutycycle (merged) on one face and 30% duty cycle on the other (data set401), and a substrate with 70% and 30% duty cycles on the two faces,respectively (data set 402). The optimum conversion efficiency region issubstantially smaller for the 100/30 duty cycle substrate, indicated bythe separation 403 between the two horizontal lines 404 and 405 becausethe domain changes size more from the top to bottom surfaces than in the70/30 duty cycle substrate, where the region is represented by theseparation 406 between the two horizontal lines 407 and 408.

In another embodiment of the present invention, the taper of the domainis controlled in order to increase the dimension of the domain on theback face while maintaining good domain quality on the front face andincreasing the size of the optimum conversion efficiency region. Thiscan be achieved by applying a voltage pulse or series of pulses to thecrystal after the illumination is removed.

The substrate can be prepared as shown in FIG. 6, and as described inthe previous embodiment. For the poling process, the electrical contactto the metal electrode is made by probe contacts 610 and the transparentelectrode 605 on the back face is created, for example, with a solutionof lithium chloride in de-ionized water. The liquid can be confinedusing an O-ring 604 with a quartz cover plate 611, or a simple tapecutout or a silicone gel or grease barrier as described previously.

The experimental setup of FIG. 8 may be used for the poling. Thecomputer 805 controls the high voltage pulsed power supply 806 and theUV/visible light source 804. The electrical contacts 808 to the crystal801 are connected to the high voltage power supply, and a current sensor809 is preferably incorporated into the circuit. Preferably, the polingsystem is controlled by the computer, which can capture the voltage,current and charge flow data in real time, allowing different voltagesto be sequenced or triggered or shut down based on time, current flow,or charge transfer values, or any combination of these. Again,preferably, a photodiode or optical monitor 807 is incorporated into thepoling fixture in order to monitor the illumination source so that thecomputer control program can also sequence the required illuminationexposure. Preferably, the computer controls the voltage supply, lightsource shutter and timing sequence. Alternatively, some features such asthe illumination time may be controlled by independent timers and theoptical monitor used to provide process sequencing via the PC.

The light output from the light source is arranged to provide sufficientillumination intensity and uniformity across the electrical contactarea. Typical intensities of ˜10 W/cm² (broadband, such as allwavelengths from a high pressure mercury bulb, for example) at theoutput of the light pipe are used. As previously described, a beamshaping/expanding system can be used to increase the illumination beamdiameter and/or uniformity on the substrate. Typically, about 0.5 W/cm²of total light intensity is incident at the surface the substrate.Higher and lower intensities may be used with the appropriateadjustments in illumination time and applied voltage to achieve domaininversion.

A typical sequence of voltage and UV light poling is shown in FIG. 10.An initial voltage of about 2000V (˜4000V/mm) 1001 is applied to thesubstrate in the absence of illumination, ramping up from zero volts inabout 60 ms. At this time no poling occurs because the voltage issignificantly below the coercive field required to achieve domaininversion, and therefore no current flows in the poling circuit. Theshutter of the illumination source is then opened, illuminating theunpatterned face of the substrate with both visible and ultra violetwavelengths 1010. It is also possible to use a combination of one ormore narrowband light sources to achieve the same effect, as long as thewavelength(s) and intensity(ies) are chosen so as to produce a similarquantity and distribution of photo-excited charges within thesubstrate.) After the illumination begins, a photo-induced current 1020starts to flow through the substrate under the influence of the highvoltage applied across it. This current generally increases graduallyover a time frame of 100ths to 10ths of seconds, and then tends to reacha plateau. Once the photo-current has increased to a sufficient value,which is determined largely by the area of exposure and the appliedvoltage, domain inversion can occur within the substrate despite theapplied electric field being nominally below the coercive field. Thisdomain inversion appears to be seeded from the patterned face and isthought to result at least in part from the effects of the concentrationof the photo-induced current flow in the small features of the patternedmetal electrode. It is thought that domain inversion occurs even thoughthe applied voltage is significantly below the nominal coercive field ofthe material at least in part because the mobile charges generated inthe substrate by the illumination decrease the effective coercive fieldof the illuminated material.

The function of this illuminated voltage pulse (i.e., the combination ofvoltage 1001 and illumination pulse 1010) is to seed or initiate thedomain inversion, so the illumination is terminated before the poling iscomplete. This termination can be based on an empirically determinedtime or a charge flow monitored by the computer, at which point thelight source shutter is closed and the illumination is blocked. Typicalvalues for this first pulse are a duration of ˜0.5 to 1 sec, and acharge flow of 0.02 to 0.12 milllcoulombs per square centimeter (mC/cm²)at a voltage of ˜2000V (4000V/mm) and an illumination intensity of ˜0.5W/cm².

Once the initiation of the domain inversion in the illuminated voltagepulse is performed, the illumination light is shut off and, using theoptical monitor 807 for sequencing control, the computer applies asecond voltage pulse (post-illumination voltage pulse) 1002. Preferably,this post-illumination voltage pulse is higher in magnitude than thatused during the illuminated voltage pulse, since there are nophoto-excited charges being generated to decrease the coercive field ofthe material. Typically a voltage of around 3000-4000V (6000-8000V/mm)may be applied post illumination. During this post-illumination voltagepulse, the poling current 1021 typically increases to a well definedpeak 1022, and then decreases to a plateau value 1023. The decrease ofthe poling current is related to the completion of the domain inversion.If the voltage is removed while the current is at the peak, the polingpattern will typically be under duty cycle and some domain features willbe incomplete. If the voltage is maintained until the poling current hasdecreased to its plateau value the domain pattern will typically becomplete, with a duty cycle on the front (patterned) face of thesubstrate that is dependent on the parameters of the illumination andillumination voltage pulse. Maintaining the voltage for a significantlength of time after the current has decreased to its plateau valuetypically leads to over duty cycle domains and a larger number of mergeswithin the domain inversion pattern. The post-illumination voltage pulsemay be controlled using the computer control program based on either thecharge flow 1030 within the circuit or the value and gradient of thepoling current 1021 or on a combination of both. Thus, poling may beterminated when a particular charge has been transferred, when thecurrent has fallen to a particular value, when the rate of decrease ofthe current reaches a certain value or any combination of these (andother) parameters.

After electric field poling, the insulating layer 607 over the metalelectrode is stripped off and the metal electrode is etched off. Thesubstrate may then be etched in HF to reveal the poled pattern. Theinverted domains are generally observed on the back face 610 of thesubstrate. Tailoring of the dose of illumination, the voltage appliedwhen the illumination is applied, the post-illumination voltage andpulse duration, etc, can be used to adjust the duty cycle of the domaingrating and the taper angle of the domain from the front surface to theback surface.

After etching of the front and back faces of the substrate to reveal thedomain inversion patterns, the quality of the domain inversion gratingdevice can be estimated using FIG. 4. The wafer substrate containing thedomain inversion gratings may be diced to separate the individualgratings, which may have different periods corresponding to differentpatterns on the photolithographic mask. The end faces of each device maythen be optically polished and, preferably, coated with anti-reflectioncoatings, ready for use as a quasi-phase-matched frequency conversiondevice as shown in FIG. 5. As described above, the position of theoptical beam within the crystal can be adjusted in depth to utilize theoptimum 50/50 duty cycle region of the grating which results from thetapered domain structure. In this embodiment, the domain taper angle isreduced compared to that of the process of the earlier describedembodiment. This means that there is less dimensional variation in thedomains from the front face to the back face of the crystal. Thisresults in a wider optimal efficiency region within the crystal (i.e., agreater depth range over which the duty cycle is within some percentageof 50/50), but requires more control to be exercised over the dimensionof the domain on the front face to ensure that the 50/50 duty cycleregion is centrally located within the crystal.

Optimization of the domain grating quality, duty cycle and taper doesnot have to be limited to the simple sequence of one illuminated voltagepulse followed by a second higher voltage pulse. Any sequence ofilluminated and un-illuminated voltage pulses may be used in any orderto provide the required poling charge to realize the desired domaininversion pattern in the substrate independent of the presence of anyphoto-current due to the illumination.

Voltage pulses can be simultaneous with illumination pulses, voltagepulses can precede or follow illumination pulses, voltage pulses can belonger or shorter than illumination pulses. Time delays may be appliedbetween the termination of one illumination or voltage pulse and theapplication of the next. In addition, different illumination spectra(light wavelengths) may be used in different illumination pulses withany combination of different applied voltages.

The metal electrode may also be patterned on the −z face, depending onthe type of substrate. In general, adjustments of the pulse parameters(such as the direction of the applied illumination, and the magnitudeand sequence of the illumination and applied voltages) compared to thoseused for a +z face patterned crystal will be required to achieve optimaldomain inversion patterns.

Because different wavelengths are absorbed in the material at differentdepths, it is possible to use a time-varying illumination wavelength toproduce a variation with time in the depth at which charges aregenerated within the substrate. In particular, a rotating circularfilter where different cut off wavelengths are coated along the circularpath may be used to change the illumination wavelength with time duringthe voltage pulse. A suitable profile of illumination wavelength versustime and, therefore, of charge generation depth, will help guide thedomain growth through the bulk of the substrate from the patterned faceto the un-patterned face.

Alternatively, a series of fixed wavelength filters may be steppedacross the illumination beam in turn to alter the wavelength spectrumincident on the substrate. Preferably, the time taken to introduce orremove the filter from the beam should be short in comparison to thetotal illumination time so that the transition of the edge of the filteracross the beam does not affect the illumination uniformitysignificantly.

In another embodiment of the present invention, a dielectric currentmask with a liquid contact electrode is used, as shown in FIG. 11. Forperiodic domain inversion in, for example, a 0.5 mm thick ˜5% MgO dopedcongruent lithium niobate substrate 1101, a patterned insulating mask1102 may be used as described in U.S. Pat. No. 5,800,767 and U.S. Pat.No. 5,519,802. Preferably the mask is applied to the −z face of thesubstrate (although the +z face can also be used) and consists of alayer of photoresist (e.g. Shiply 3312 or AZ 4210) some 2-4 micronsthick. After spinning onto a clean MgO:CLN wafer and softbaking (e.g.,˜90 C for 30 minutes), the photoresist is exposed using standardphotolithographic techniques (e.g., contact or projection lithography)and developed to produce the pattern desired for the domain inversiongrating. After ensuring the removal of all photo resist residue from thepattern openings the resist layer 1102 is hard-baked, preferably at atemperature of around 120° C. or higher. The hard-bake temperature ischosen as a trade-off between crosslinking of the photoresist andslumping or distortion of the photoresist pattern during the bakeprocess, which is undesirable for the subsequent electric field polingprocess. It should be noted that different bake times and temperatureswill be applicable to different resist formulations and thicknesses anddifferent patterns, and should generally be chosen to provide asubstantially electrically insulating layer on the surface of thecrystal wafer.

Electrical contact to the crystal surface during the poling process maybe made using a conductive liquid 1103, such as a solution of lithiumchloride in de-ionized water, to form liquid electrodes. The conductiveliquid 1103 is preferably applied to the front, patterned face 1105first and may be confined to the desired contact area using an o-ringand a quartz cover plate or a simple tape cut-out (not shown) asdescribed above. Restricting the contact area of the liquid ispreferable in order to ensure the uniformity of the poling process.UV/visible illumination 1104 is incident from the back, unpatterned face1106 of the substrate. The dimensions of the poling area where theliquid contact is made should preferably match or be less than thedimensions of the area that can be uniformly illuminated by theavailable light source. If the contact is applied over regions that arenot uniformly illuminated, the resulting domain inversion pattern willgenerally be non-uniform. It should be noted that the electrical contactareas on the front and back faces of the substrate do not have to be thesame size. For instance, if electrical contact is made to the entirefront face of the substrate at once, the poling area may be defined to asmaller area by confining the liquid electrode on the back face, andpreferably the illuminated area, to a small subset of the substratesurface (e.g., using a UV-opaque dicing tape to confine the liquidconductor and cover the remaining portions of the back face of thesubstrate). Electrical contact between an external circuit and theliquid conductor on the front face of the substrate may be made byplacing the substrate front-face-down onto a metal contact plate.Connection to the liquid electrode on the back face of the substrate maybe made with one or more probe wires, positioned to allow uniformdistribution of voltage and current to the poling area while notobstructing the illumination of the substrate.

It is important to ensure that good electrical contact is made to thesubstrate surface by the liquid conductor. This may be achieved byadding a small amount of a surfactant to the liquid to reduce thesurface tension, allowing it to more readily wet the small features inthe photoresist pattern on the front face. Alternatively, thephotoresist pattern may be overcoated with a conductor (e.g., bysputtering a metal or carbon conductive layer), so that electricalcontact is maintained from the top of the mask down to the substratesurface without the need for the liquid conductor to completely filleach feature in the pattern.

The experimental setup of FIG. 8 may be used for the poling. Thecomputer 805 controls the high voltage pulsed power supply 806 and theUV/visible light source 804. The electrical contacts 808 to thesubstrate 1101 are connected to the high voltage power supply with thecurrent sensor 809 preferably incorporated into the circuit. Preferably,the complete poling system is controlled by the computer, which cancapture the voltage, current and charge flow data in real time, allowingdifferent voltages to be sequenced or triggered or shut down atdifferent times, current flows, or charge transfer values, or anycombination of these. Again, preferably a photodiode or optical monitor807 is incorporated into the poling fixture in order to monitor theillumination source so that the computer control program can alsosequence the required illumination exposure. Preferably, the computercontrols the voltage supply, light source shutter and timing sequence.Alternatively some features such as the illumination time may becontrolled by independent timers and the optical monitor used to provideprocess sequencing via the computer.

The light output from the light source 804 coupled through light pipe803 and beam shaping/expanding system 802 is arranged to providesufficient illumination intensity and uniformity across the electricalcontact area. Typical broadband intensities of ˜10 W/cm² at the outputof the light pipe may be used as described above. Typically, about 0.5W/cm² of total light intensity is incident at the surface the substrate.Higher and lower intensities may be used with the appropriateadjustments in illumination time and applied voltage to achieve domaininversion.

The voltage and illumination sequence illustrated in FIG. 10 may also beused for the poling process in this embodiment. An initial voltage 1001of about 2000V (˜4000V/mm) is applied to the substrate in the absence ofillumination. At this time no poling occurs because the voltage issignificantly below the coercive field required to achieve domaininversion, and therefore no current flows in the poling circuit. Theshutter of the illumination source is then opened, illuminating theunpatterned face of the substrate with both visible and ultra violetwavelengths 1010. As described above, it is also possible to use acombination of one or more narrowband light sources to achieve the sameeffect, as long as the wavelength(s) and intensity(ies) are chosen so asto produce a similar quantity and distribution of photo-excited chargeswithin the substrate.) After the illumination begins, a photo-inducedcurrent 1020 starts to flow through the substrate in response to thehigh voltage applied across it. This current generally increasesgradually over a time frame of 100ths to 10ths of seconds, and thentends to reach a plateau. Once the photo-current has increased to asufficient value, which is determined largely by the area of exposureand the applied voltage, domain inversion occurs within the substratedespite the applied voltage being nominally below the coercive field.

As previously describe, the function of the illuminated voltage pulse(i.e., the combination of voltage 1001 and illumination pulse 1010) isto seed or initiate the domain inversion, so the illumination isterminated before the poling is complete. In general, the domainsresulting from this illuminated voltage pulse are tapered, and theirwidth on the patterned face and depth into the substrate both generallyincrease with increasing voltage, increasing illumination intensity andincreasing illumination time. This termination can be based on anempirically determined time or a charge flow monitored by the computer,at which point the light source shutter is closed and the illuminationis blocked. Typical values for this first pulse are a duration of ˜0.5to 1 sec, and a charge flow of 0.02 to 0.12 mC/cm² at a voltage of˜2000V (4000V/mm) and an illumination intensity of ˜0.5 W/cm².

The domain shape and size may be further controlled and the quality ofthe domain inversion grating structure enhanced by applying a furthervoltage pulse 1002 after the illumination is removed. Preferably, thispost-illumination voltage pulse is higher in magnitude than that usedduring the illuminated voltage pulse, since there are no photoexcitedcharges being generated to decrease the effective coercive field of thematerial. Typically a voltage 1002 of around 3500V (7000V per mm) may beapplied post illumination. During this un-illuminated voltage pulse, thepoling current 1021 typically increases to a clearly defined peak 1022,and then decreases to a plateau value 1023. The decreasing polingcurrent is related to the completion of the domain inversion. If thevoltage is removed while the current is at the peak, the poling patternwill typically be under duty cycle and some domain features will beincomplete. If the voltage is maintained until the poling current hasdecreased to its plateau value the domain pattern will typically becomplete, with a duty cycle that is dependent on the parameters of theillumination and illumination voltage pulse. Maintaining the voltage fora significant time after the current has decreased to its thresholdvalue typically leads to over duty cycle domains and a larger number ofmerges within the domain inversion pattern. The post-illuminationvoltage pulse may be controlled using the computer control program basedon either the charge flow 1030 within the circuit or the value andgradient of the poling current 1021 or on a combination of both. Thus,poling may be terminated when a particular charge has been transferred,when the current has fallen to a particular value, when the rate ofdecrease of the current reaches a certain value or any combination ofthese (and other) parameters.

After poling, the insulating mask layer 1102 is stripped off of thesubstrate front face 1105. The substrate may then be etched in HF toreveal the poled pattern. The inverted domains are generally observed onthe back face 1106 of the substrate. Tailoring of the dose ofillumination (illumination time and intensity), the voltage applied whenthe illumination is on, the post-illumination voltage and pulseduration, etc can be used to adjust the duty cycle of the domain gratingand the taper angle of the domain from front surface to the backsurface.

After etching of the front and back faces of the substrate to reveal thedomain inversion patterns, the quality of the domain inversion gratingdevice can be estimated using FIG. 4, as described above. The substratecontaining the domain inversion gratings may be diced to separate theindividual gratings, which may have different periods corresponding todifferent patterns on the photolithographic mask. The end faces of eachdevice may then be optically polished and, preferably, coated withanti-reflection coatings, ready for use as a quasi-phasematchedfrequency conversion device as shown in FIG. 5. The position of theoptical beam within the substrate can be adjusted in depth to utilizethe optimum 50/50 duty cycle region of the grating which results fromthe tapered domain structure. In this embodiment, the domain taper angleis also reduced, so that there is less dimensional variation in thedomains from the top face to the bottom face of the substrate comparedwith the poling process described in connection with FIG. 9. Thisresults in a wider optimal efficiency region within the crystal (i.e. agreater depth range over which the duty cycle is within some percentageof 50/50), but requires more control to be exercised over the dimensionof the domain on the top face to ensure that the 50/50 duty cycle regionis centrally located within the crystal.

As described above, optimization of the domain grating quality, dutycycle and taper does not have to be limited to the simple sequence ofone illuminated voltage pulse followed by a second higher voltage pulse.Any sequence of illuminated and un-illuminated voltage pulses may beused in any order to provide the required poling charge to realize thedesired domain inversion pattern in the substrate independent of thepresence of any photo-current due to the illumination.

Voltage pulses can be simultaneous with illumination pulses, voltagepulses can precede or follow illumination pulses, voltage pulses can belonger or shorter than illumination pulses. In addition, differentillumination spectra (light wavelengths) may be used in differentillumination pulses with any combination of different applied voltages.

The insulating mask may also be patterned on the +z face, depending onthe type of substrate. In general, adjustments of the pulse parameters(such as the direction of the applied illumination, and the magnitudeand sequence of the illumination and applied voltages) compared to thoseused for a −z face patterned crystal will be required to achieve optimaldomain inversion patterns.

In another embodiment of the present invention, patterned current flowis generated by a combination of a patterned illumination and apatterned applied electric field. For instance, the substrate may bepatterned with an electrically insulating and optically absorbing orreflecting masking material. This mask can simultaneously provide thedual roles of patterning the illumination and the applied electricfield. The substrate is illuminated from the masked face, resulting inonly the open areas in the mask pattern being illuminated and thusmobile charges being generated only in those areas of the substrate.Preferably, the illumination wavelength(s) are chosen such that thepenetration depth into the illuminated regions of the substrate isshort, such that no substantial diffraction or interference pattern canresult in the substrate which otherwise would allow charge generation inunwanted areas of the substrate. Simultaneously with the illumination,the electric field is patterned by the insulating mask such that theareas of the substrate in the open areas of the mask pattern aresubjected to a high electric field, while the field in the areas coveredby the mask is lower. The combination of patterned illumination andpatterned electric field results in a patterned photocurrent flow in thematerial which provides enhanced seeding for domain inversion at thepatterned face.

FIG. 12 an example of this embodiment where a substrate 1201 of, forexample, 0.5 mm thick z-cut 5% MgO doped congruent LiNbO₃ is used.Preferably, the patterned electrode is defined on the −z face of thesubstrate. An insulating layer 1202 is deposited onto the −z face of thesubstrate, for example a photoresist layer of ˜2 μm thickness, or aspin-on-glass layer of ˜1 μm thickness. The insulating layer ishard-baked to crosslink the material and to provide robust physical andelectrically insulating properties. Since an insulating layer likephotoresist can absorb UV light and generate photo-induced charges, itis preferable to provide a metal over-layer 1203 that blocks the UV andvisible light from reaching the photoresist mask. A metal layer such asTi, NiCr, Al etc, preferably with a high absorption and or reflection inthe UV and visible spectrum, is deposited on the surface of theinsulator (e.g., by evaporation or sputtering). A layer of photoresist1204 is spun on the metal layer and standard photolithographicpatterning processes are used to define the desired electrode pattern.The pattern may be transferred into the metal layer using a wet or dryetch (e.g., reactive ion etching or sputter etching). The underlyinginsulating layer of hard-baked resist or spin-on-glass is then patternedto provide openings to the substrate surface, preferably using areactive ion etch process to create substantially vertical walls and toavoid damage to, or removal of the metal light blocking layer.

Electrical contact to the substrate surface during the poling process iseasily made using a liquid conductor (electrolyte) 1207, such as alithium chloride solution, for example, that functions as a liquidelectrode. The electrolyte 1207 is preferably applied to the patternedface first and may be confined to the desired contact area using ano-ring or a simple tape cut-out. It is important to ensure that goodelectrical contact is made to the substrate surface by the electrolyte.This may be achieved by adding a small amount of a surfactant to theelectrolyte to reduce the surface tension, allowing it to more readilywet the small features in the photoresist patter.

Electrical contact to the opposite (back) face of the substrate (theunpatterned or back face) is made in a similar way with a liquidelectrode 1205 or, alternatively, may be achieved with metallization ofthe back face of the substrate. The substrate is oriented with thepatterned face facing the output of the UV/visible illumination source.Contact to the liquid or metallic electrode on the unpatterned back facemay be made with a simple probe in the liquid contact or to the metalelectrode. Contact to the patterned front face may be made using a probecontact to the edge of the liquid conductor 1207 so as not to block theillumination from entering the substrate.

The poling sequence may be described as follows and as illustrated inFIG. 10. A voltage pulse 1001 of about 2000 volts (˜4000V/mm) is appliedacross the electrodes 1207 on the front surface and 1205 on the backsurface of the substrate. During this pulse, a UV light pulse(illumination pulse) 1010 is applied to the patterned front surface ofthe substrate. A photo-current 1020 starts to flow through the substratedue to the photo-induced charges created by the illumination which movein response to the applied external field. The illumination pulse isapplied for about 0.3 seconds, then the UV light is shut off. Thecombination of illumination and applied voltage induces seeding of thedomain inversion pattern, despite the applied voltage being considerablybelow the coercive field of the bulk substrate material. The seedingoccurs only in the open areas of the mask on the −z face where theapplied field is high and the illumination reaches the surface of thesubstrate, thereby allowing a patterned photocurrent to flow in thoseconfined regions. In general, at the end of the illumination pulse, theseeded domain inversion features are terminated within the bulk of thesubstrate and do not extend all the way to the unpatterned electrode onthe back face of the substrate.

Once the light pulse is terminated, a second high voltage pulse(post-illumination voltage pulse) 1002 is applied to the substrate togrow the domains through the substrate. Typically the voltage of thesecond pulse is around 3500V (˜7000V per mm). During thispost-illumination voltage pulse, the poling current 1021 generallyincreases to a clearly defined peak 1022, and then decreases to aplateau value 1023. The decreasing poling current is related to thecompletion of the domain inversion. If the voltage is removed while thecurrent is at the peak 1022, the poling pattern will typically be underduty cycle and some domain features will be incomplete. If the voltageis maintained until the poling current has decreased to its plateauvalue 1023, the domain pattern will typically be complete with a dutycycle that is dependent on the parameters of the illumination andillumination voltage pulse. The post-illumination voltage pulse may becontrolled using a computer control program, as described above inreference to the experimental setup of FIG. 8, based on either thecharge flow 1030 within the circuit or the value and slope of the polingcurrent 1021 or on a combination of both.

The choice of wavelength in this embodiment is dictated by theconsideration that the light should be absorbed close to the surface ofthe material. The absorption is preferably strong so as to preventsignificant diffraction or the creation of an interference pattern inthe bulk of the material. In the absence of diffraction, the photocurrent will be well-defined by the opening in the insulating mask layerand therefore the domain inversion seeding and subsequent growth will besimilarly well-defined, creating the desired domain inversion patter.Preferably, the absorption depth of the illumination is a few micronsinto the substrate.

In another embodiment of the present invention, a coherent (i.e., singlewavelength) UV or visible light source is used as the illuminationsource, enabling an interference pattern to be created within thesubstrate. The coherent light source may be a frequency doubled diodepumped solid state laser or gas laser such as an argon or krypton ionlaser, or any other laser source operating in the UV/visible spectralregion. Absorption of light at the constructive interference fringeswithin the substrate generates localized concentrations of photo-inducedmobile charges. These charges form a photo-current in response to avoltage applied across the faces of the substrate, and thisphotocurrent/voltage combination is used as previously described to seeddomain inversion in a localized manner.

In FIG. 13, an unpatterned MgO:CLN substrate 1301 of 0.5 mm thickness isillustrated. A prism 1305 (e.g., fabricated from BK7 borosilicateoptical glass) is used to split an incoming light beam 1302 (e.g., froma Krypton ion laser @ 413.1 nm) and to create an interference pattern1303 with the desired period within the substrate. The angle of theprism 1306 for BK7 glass is designed to be 5.043 degree to generate a4.425 um grating period. It may be preferable to use LiNbO3 as the prismmaterial, and the angle for LiNbO3 will be 1.891 degrees. The incidentangle of the input light beam to the prism must be well controlled, andthe orientation of the substrate must be accurately set relative to theaxes of the interference fringes such that the fringes lie substantiallyalong the y-axis of the substrate.

An optically transparent conducting liquid 1304 such as LiCl in watermay be used as an electrode and is introduced between the prism and thesubstrate surface, preferably forming a smooth, continuous layer with nobubbles or thickness variations that can affect the uniformity of theinterference pattern. If desired, pressure can be applied to the prismto ensure that the liquid layer is thin and uniform. Alternatively, atransparent conductor such as ITO (indium tin oxide) may be deposited onthe substrate surface or the surface of the prism to act as theelectrode.

Electrical contact to the liquid electrode may be made at the edge ofthe prism. Typically a voltage of ˜2000V (˜4000V/mm) is applied to thesubstrate while it is being illuminated, and the photocurrent flowthrough the substrate is monitored as described above. Preferably, thevalue of the photocurrent is kept low so that the current flow isstrongly localized to the narrow constructive interference regions ofthe interference pattern.

After allowing the photocurrent to flow for a period of time varyingfrom seconds to minutes, depending on the magnitude of the appliedvoltage, the intensity of the illumination, the magnitude of thephotocurrent and the poled area and material type of the substrate beingpoled, a higher voltage is applied to complete the domain inversion. Itis preferable to block the illumination as the higher voltage isapplied, to prevent a dramatic increase in photo-current flow. Thehigher, poling growth voltage is generally of the order of 3500-4000Vand may be applied either as a step function or continuously ramped fromthe initial to final values. A current sensor may be used to monitor thecharge flow during the poling pulse and, accounting for the photocurrentflow, the poling pulse may be terminated when sufficient charge has beentransferred to achieve the desired amount of poling.

When choosing an illumination wavelength for this embodiment it isnecessary to consider the dual requirements of a reasonably strongabsorption to generate the necessary photo-induced charges whilesimultaneously allowing the interference fringes to extend to asubstantial depth into the substrate. Therefore the illuminationwavelength is preferably in the long wavelength UV to short wavelengthvisible range around approximately 400 nm, considerably above the bandedge of ˜320 nm. As the optical absorption will lead to a gradient inphoto-charge density with depth into the substrate, there may be apreferential illumination direction. For example, it may be preferableto illuminate the substrate through the +z face.

In the above described process the inverted domains are seeded in thenarrow illuminated regions of constructive interference in the opticalinterference pattern. However, in the arrangement of FIG. 13 there is nomechanism to prevent the domains from growing laterally, to form theenergetically favorable hexagonal domain shape, other than thepreferential seeding and poling due to the localized photo-current andillumination. In practice, the domain confinement provided by thelocalized photo-current and illumination is not sufficient to preventthe domains from expanding laterally and merging together at shortperiods.

An improvement to this embodiment is illustrated in FIG. 14, where apatterned insulating mask layer 1404 is provided on the opposite face ofthe substrate to the illuminating beam 1402. The period of the patternin the insulating mask is the same as that of the interference patternwithin the substrate, and the mask is preferably aligned such that thedomain inversion features run substantially parallel to the y-axis ofthe substrate. The substrate 1401 is mounted on a rotation andtranslation stage (not shown) so that alignment can be achieved betweenthe patterned mask layer and the optical interference pattern. Thesubstrate is illuminated from the unpatterned face to create an opticalinterference pattern. A lens (not shown) is placed adjacent to thepatterned face of the substrate to collect the light that is transmittedthrough the crystal and transfer it to a photodetector. When theconstructive interference fringes of the interference pattern arealigned with the openings in the insulating mask in both rotational andtranslation directions, the observed transmitted light signal will reacha maximum. Once alignment is achieved, it may be preferable to block thecoherent light to remove the interference pattern and allow thephoto-generated charges created during the alignment procedure todissipate.

With the insulating mask and interference pattern aligned, a similarpoling sequence to that described above for the unpatterned sample canbe performed. With the illumination source incident on the substrate, avoltage is applied to liquid electrodes 1405 on the patterned face andunpatterned face, resulting in a photo-current flow which is nowconfined by both the constructive interference regions and the openingsin the insulating mask aligned to the interference pattern. The extracurrent confinement effect of the insulating mask combines with amodulated electric field to improve the definition of the domain patternand prevent unwanted lateral expansion of the domains.

The patterned insulating mask layer also provides a further benefit forthe short period domain inversion process. As discussed earlier withreference to FIG. 7, there is a maximum length of poling feature whichis preferable for a given period due to lateral domain expansion as aresult of angular misalignment between the domain feature and thecrystal axes of the substrate. In the present embodiment, the opticalinterference pattern is composed of fringes which are continuous acrossthe entire illuminated area. Thus, any slight misalignment will causelateral expansion based on the full width of the poled area, easilycausing merging of short period gratings. However, the patternedinsulating mask layer enables the continuous fringes to be effectivelybroken into shorter lengths by adding an insulating barrier to blockdomain inversion at certain points along the length of the fringe.

Thus, the length of the domain features can be photolithographicallyreduced to the ≦230 μm length preferable for a 4 μm period grating, or≦180 μm for a 3 μm period grating, based on an angular misalignmenttolerance of ˜0.5 degrees between the grating and the crystal axes ofthe substrate.

Another embodiment of the invention is shown in FIG. 15. An alternativeapproach to generating the mobile charges of the present invention is touse a high energy electron beam to inject the charges into thesubstrate. The combination of the uniform electron beam irradiationthrough the back face of the substrate with the patterned metal orinsulating mask on the front face results in patterned current flow atthe patterned mask surface and through the depth of the substrate,resulting in patterned domain inversion. This greatly simplifies theequipment requirements compared to the prior art focused electron-beamapproach to domain inversion.

When using a high energy electron beam for domain inversion according tothe present invention, the energy of the electron beam can be varied intime from high to low or low to high to vary the penetration depth ofthe electrons into the substrate. This capability in principle providesadvantages over the illuminated embodiments of the present inventionsince the electron beam energy and penetration depth can be moreflexibly and tightly controlled than the absorption of light, which islimited by the available wavelength spectrum and the absorption spectrumof the material.

An external voltage may be applied to the substrate in a similar mannerto the illuminated embodiments, and may be applied before, during andafter the electron bombardment. It should be noted that when the highenergy electrons are stopped inside the substrate, the kinetic energy ofthe electrons will be absorbed and increase the temperature of thesubstrate, which may decrease the coercive field for the domaininversion process.

FIG. 15 illustrates a substrate 1501, preferably 0.5 mm thick MgO:CLN,with a patterned metal electrode 1502 disposed using standardphotolithographic deposition and patterning techniques on the +z face ofthe substrate. Alternatively a patterned insulating mask may be used. Athin metal electrode 1503 (e.g., ˜1000A of titanium) is deposited on the−z, or back, face of the substrate. A high energy electron beam system(e.g., a system from HVEA Inc, not shown) is used to generate anelectron beam 1505 which is incident on the unpatterned −z face of thesubstrate. The electron energy and dose are controlled by theaccelerating voltage and current flow of the electron beam systemrespectively.

The electron beam is collimated to provide uniform exposure over adefined area, preferably over the entire substrate surface. For electricfield poling, the electron beam is incident through the unpatterned −zface while the patterned +z face is grounded. The electric fieldgenerated by the accumulated electrons within the substrate is generallysufficient to cause domain inversion to occur. An external voltage maybe applied between the patterned electrode 1502 and the unpatternedelectrode 1503 to control the flow of mobile-charge-current through thesubstrate and improve the domain inversion pattern definition andquality.

The improved domain inversion structure of the present invention is ofparticular value when used to construct a frequency converter for theapplication of second harmonic generation for the creation of visiblelaser light sources. This application has proven very challenging forprior art devices due to the difficulty in fabricating the very shortgrating periods required, ˜4 μm to 6 μm, with high quality anduniformity, and due to the performance degradations due tophotorefractivity and green and blue induced infra-red absorption(GRIIRA and BLIIRA). The present invention provides a fabricationprocess for high quality, high uniformity and high efficiencyquasi-phasematched frequency converters with periods as short as 4 μm ina photorefractively robust material, MgO:CLN. The present invention alsoprovides a high temperature annealing process coupled with a closed loopdischarge path which enables the effects of BLIIRA and GRIIRA to besignificantly reduced. In addition, the present invention provides anoptimized frequency conversion device with a tapered domain structurewhich ensures that at least some portion of the bulk crystallinesubstrate has an optimum 50/50 duty cycle domain grating.

In one embodiment of the present invention, an efficient visiblefrequency conversion element and device is provided for the generationof visible light using an improved domain inversion structure. Such adevice is shown schematically in FIG. 16, indicating a single passfrequency doubled laser system. A pump laser 1602 is coupled into afrequency converter 1601 using a set of coupling optics 1604 (e.g., afocusing or collimating lens). The input face of the frequency converteris preferably anti-reflection (AR) coated at the fundamental wavelengthso as to minimize efficiency losses in the optical conversion process,while the output face is preferably AR coated for at least the secondharmonic wavelength and preferably for the fundamental wavelength aswell. The pump laser 1602 may be a semiconductor diode laser, a diodepumped solid state laser (e.g., Nd:YAG or Nd:YVO₄), a gas laser or anyother type of laser with coherent output light 1603 of fundamentalfrequency which matches the conversion wavelength of the frequencyconverter 1601. Preferably, for efficient frequency conversion, thespectral bandwidth of the laser pump source is less than or comparableto the phasematching bandwidth of the frequency converter. Thepolarization of the pump beam is preferably arranged to be parallel tothe crystal z-axis of the frequency converter to enable the highestnonlinear coefficient, d₃₃ to be used for the frequency conversioninteraction.

The temperature of the frequency converter 1601 is generally adjustedusing a heated mount 1605 so as to match the operating wavelength of theconverter with the input fundamental pump laser wavelength.Alternatively, the wavelength of the pump laser may be tuned using agrating or an etalon (i.e., a Fabry-Pérot interferometer) so that itmatches the acceptance wavelength of the frequency converter. Anadvantage of the MgO:CLN devices enabled by the present invention isthat the operating temperature is much lower than that required for CLNdevices, less than 100° C. versus greater than 200° C. The position ofthe pump beam 1603 within the frequency converter 1601 should beadjusted for maximum conversion efficiency to make use of the 50/50 dutycycle of the domain grating which is ensured by the tapered domainstructure. An optical filter 1606 may be located in the output beam toremove the residual pump beam and transmit the second harmonic output atvisible or UV wavelengths.

In the second harmonic generation application described here, therequired period of the domain inversion grating in the frequencyconverter is determined by the wavelengths of the interacting beams asfollows:

$\Lambda = \frac{\lambda_{pump}}{2\left( {n_{sh} - n_{pump}} \right)}$where Λ is the grating period, λ_(pump) is the pump wavelength, n_(sh)is the refractive index at the second harmonic wavelength and n_(p) isthe refractive index at the pump wavelength.

A knowledge of the refractive index dispersion and thermo-opticcoefficients of the nonlinear optical crystal enables a domain inversiongrating to be designed to quasi-phasematch a second harmonic interactionat a particular wavelength and at a particular temperature.

The frequency conversion device of FIG. 16 finds applications in medicalinstrumentation, semiconductor metrology and laser display devices.

An alternative application for the frequency conversion device of thepresent invention to yield efficient second harmonic generation isintra-cavity frequency conversion as shown in FIG. 17. Placing afrequency converter 1702 inside a pump laser cavity makes use of thehigher circulating power within the cavity compared to the CW or pulsedoutput from the same laser, significantly increasing the efficiency ofthe second harmonic generation interaction, which is proportional to thesquare of the power in the pump beam. The laser gain medium 1701 may bean electrically pumped semiconductor laser, an optically pumpedsemiconductor, or an optically pumped crystal gain medium such asNd:YAG. One face of the gain medium 1701 is high reflection coated atthe fundamental pump wavelength, and the other face is anti reflectioncoated for the fundamental. Preferably, this face is high reflectioncoated at the second harmonic wavelength to prevent the second harmonicvisible light from damaging the semiconductor pump source. The domaininverted frequency converter 1702 is placed adjacent to the gain medium1701. Optionally, there may be coupling optics such as a focusing orcollimating lens (not shown) disposed between the gain medium and thefrequency converter. Both input and output faces of the frequencyconverter are anti-reflection coated for both the fundamental and secondharmonic wavelengths. A second mirror 1703 is placed on the oppositeside of the frequency converter to the pump laser to form the resonantlaser cavity. This mirror may be a simple multilayer dielectricreflector or it may be a distributed reflector such as a volume Bragggrating. Alternatively the output face of the frequency converter may beoptically coated to form the cavity output mirror. Temperature controlof the nonlinear optical frequency converter using a heated mount 1707may be used to tune the crystal to peak efficiency for the operatingwavelength of the laser. Alternatively, the wavelength of the pump lasermay be tuned using a grating or an etalon so that it matches theacceptance wavelength of the frequency converter. Typically thefrequency conversion crystal is designed according to the relationdescribed above so that it must be held at a slightly elevatedtemperature to provide optimum conversion efficiency for the designwavelength of the laser. This is for two main reasons, firstly theslightly elevated temperature, typically 40-90° C. helps to reduce thepossibility of residual photorefraction distorting the optical beam andreducing the efficiency, and secondly maintaining the elevatedtemperature requires only a simple heater, which is generally lesscomplicated, less failure prone and more efficient than thethermoelectric cooler that would be required to provide a stableoperating temperature nearer room temperature. In addition, in consumerelectronic products, the ambient operating temperature is not wellcontrolled, so it is important to design the device to operate at atemperature higher than that which will be experienced as an ambienttemperature, so that heating is always required and a stable operatingtemperature can be maintained.

For a crystal gain medium (e.g. Nd:YAG), the laser wavelength is definedby the crystal structure energy levels and is generally well determinedand narrow band (excepting certain crystals and dopants such asTi:Sapphire which show widely tunable laser action). For semiconductorpump sources, the gain bandwidth is typically quite broad, and a furtherfrequency selective element must be provided to determine the laserwavelength and bandwidth. This frequency selective element 1705 may beplaced inside the cavity, where it may be an etalon or narrow bandfilter, or it may be incorporated into one of the two cavity mirrors inthe form of a Bragg reflection grating. Thirdly, a Bragg reflectiongrating 1706 may be deployed outside the main laser cavity to providewavelength selective feedback to the laser pump source to determine thelaser wavelength. For optimum efficiency with the frequency converter ofthe present invention, the frequency converter should be positioned suchthat the pump beam travels through the optimum 50/50 duty cycle portionof the crystal which is ensured by the tapered domain structure.

Since the conversion efficiency of the second harmonic generator isproportional to the square of the pump power, more efficient energyconversion can generally be obtained from a pulsed laser source thanfrom a CW laser. Increasing the peak and average powers too much howevercan cause crystal damage, such as surface damage at the polished faces,or residual photorefractive effects which are not compensated by the MgOdopant at very high optical powers. For this reason, for someapplications where high visible powers are required, it may bepreferable to provide an array of pump beams coupled into differentregions of the same frequency conversion chip. In this way, the power ineach individual beam can be maintained well below the material damagethresholds, while the total output power from all the beams can bescaled as high as several watts to 10 watts of power. The frequencyconverter enabled by the present invention is ideal for this applicationsince it provides high peak conversion efficiency and high lateraluniformity for uniform and efficient frequency doubling performanceacross an entire array of laser beams.

An example of an application which benefits strongly from the arrayscalability of the frequency converter enabled by the present inventionis that of laser projection displays. In this case, the fact that thetotal output power is made up of a number of individual beams is not adisadvantage, since a single mode diffraction limited optical beam isnot generally required. In fact, multiple beams each with slightlydifferent wavelengths helps to reduce the speckle effect which canotherwise render laser displays uncomfortable to watch.

An example of a laser light source for projection display applicationsis shown in FIG. 18. At present, most projection displays areilluminated by a high pressure mercury bulb. This bulb is inefficient atgenerating visible light, and the optical components required to capturea significant proportion of the light that is generated and project itonto the screen are complex and relatively expensive. In addition, acolor wheel or color filters must be used to separate the three primarycolors (red, green and blue) either spatially or temporally. A laserlight source on the other hand simplifies many of these issues andoffers some advantages for the display. Firstly the laser light sourcecan provide a wider color gamut than the lamp by producing narrowerbandwidth light at the primary colors, leading to a richer, moresaturated and natural looking color display. Secondly the opticalcoupling and projection optics for the laser beam are significantlysimpler than those for the lamp, since the laser beams are essentiallycollimated and do not need fast (wide aperture) collection optics.Thirdly the properties of the laser enable some of the other componentsto be removed from the optical system decreasing the complexity andcost. In the laser light source of FIG. 18, laser light at the threeprimary colors is generated by modules 1801, 1802, 1803, utilizingfrequency doubling of semiconductor diode lasers using the periodicallypoled MgO:CLN frequency conversion device of the present invention.Collecting optics 1804 collimate the light, overlap the multiple beamsfrom the array, and match the optical beam to the form factor of thespatial light modulators 1807, 1808, 1809, which in this embodiment aretransmission LCD panels, for instance from Epson Corp. The laser isadvantageous for the use of LCD panels since the laser light is linearlypolarized matching the optimum requirement for the LCD panel operation.The spatially modulated light at the three primary colors is combined inan x-prism 1810 and the image projected onto the screen (not shown) viathe projection optics 1811. This embodiment has described a 3-LCDprojection system wherein the use of the laser enables the eliminationof the complex collection optics required by the lamp, as well asvarious color separation filters and polarizers that are required tosplit the lamp output into linearly polarized beams at the primarycolors.

The array scaling capability of the frequency converter of the presentinvention is key to generating the power levels that are required for aprojection display. For instance, 2.0 W of 465 nm blue, 1.6 Watts of 532nm green and 2.2 Watts of 635 nm red laser light will provide 1400lumens, which after traveling through the typical spatial lightmodulators and projection optics should yield around 400 lm on thescreen. For brighter displays, even more optical power is needed,leading to the desire to reliably produce 4-5 watts of light in eachprimary color.

An alternative embodiment for the projection display uses a digitalmicro-mirror device (DMD, Texas Instruments) as the spatial lightmodulator. In this embodiment, the light output from the threeprimary-color second harmonic generation laser modules are spatiallyoverlapped before the spatial light modulator. Time sequencing of thelight output from the laser modules is used to provide color-sequentialoperation using a single spatial light modulator (SLM). Alternatively, aseparate SLM can be used for each primary color and the imagessuperimposed after the SLM. The light output from the SLM is projectedonto the screen by the projection optics. In this embodiment the laserlight sources enable the elimination of the rotating color wheel whichis currently used to provide color-sequential light output from thecontinuous wave mercury lamp, as well as simplifying the collection andprojection optics.

It should be noted that although the above projection displayembodiments have been described with reference to full color, 3-primarysource displays, it may in some cases be preferable to provide more orless than 3 primary colors. For instance, by providing 4 or 5 colors,the overall color gamut of the display can be increased and a widerrange of natural colors can be displayed. On the other hand, a simplerand cheaper display device can be provided with only a single color,producing a monochrome display with potentially much more compactdimensions and lower cost. The frequency converter of the presentinvention is particularly valuable for the small dimension and low costprojection display device, often termed the pocket projector This isbecause the frequency conversion device enabled by the present inventionhas the combined properties of highly efficient operation, highuniformity, high manufacturing yield, and fabrication in a commerciallyavailable substrate material. This enables, for the first time, theprospect of scaling the manufacturing cost of a precision designed andfabricated periodically poled nonlinear optical crystal down to the fewdollar price point required for mass volume manufacturing for consumerelectronics applications.

An alternative approach to the projection display, especially forcompact and low power devices is that of directly scanning the imageover the screen, using for instance 1 or 2 axes MEMS (micro-electricalmechanical systems) scanners. This is shown schematically in FIG. 19. Inthis case, the primary color laser modules (which generate visible lightusing the frequency converter of the present invention) 1901, 1902, 1903are imaged onto the scanning system 1904 by the coupling optics 1905.The scanning system 1904 may be composed of a single 2-axis scanner, orof a single axis scanner and a rotating faceted drum, or two single axisscanners or any other image scanning technique know in the art. Thescanner directs the color beams to the screen (not shown) and the imageis written to the screen using, for instance, raster or vector scanning.Brightness and color information is encoded by time domain modulation ofthe laser output power, either by directly modulating the pump laserpower, or by providing a modulator integrated into the frequencyconverter, or located outside the frequency conversion module. Thedevice structure and assembly are simplified by the lack of the spatiallight modulator, which also offers the prospect of lower device cost,albeit accompanied by a reduced performance in terms of brightness andimage resolution.

Another application of the improved optical frequency converter is inthe generation of infra-red light for use, for example, in remote gassensing, countermeasures and LIDAR (light detection and ranging).Infra-red wavelengths are generated with a difference frequencyconverter or an optical parametric oscillator. FIG. 20 shows adifference frequency converter using one embodiment of the presentinvention. Two pump lasers 2001, 2002 are coupled into the frequencyconverter 2003 by coupling means 2004 which may consist of one or morelenses to focus or collimate the pump laser beams. Thequasi-phasematched domain inversion grating in the frequency converter2003 transfers optical energy from the two pump lasers into a third beam2006 (which is actually generally collinear with the combined pump beam2005) with a frequency equal to the difference between the frequenciesof the pump lasers. The temperature controller 2007 controls thetemperature of the chip to provide maximum conversion efficiency. Inthis way, a pump beam from an Nd:YAG laser at 1064 nm mixes with a diodelaser at 810 nm to create a difference frequency beam at about 3.4 μm.Alternatively, the difference frequency generator can be used fortelecom wavelength conversion and dispersion compensation applications.In this case a pump beam at 775 nm is combined with a signal beam ataround 1550 nm to generate an idler beam also around 1550 nm. In thisconfiguration the signal and idler beams can range from around 1520 nmto 1570 nm due to the slow variation of the refractive index around the1550 nm wavelength in lithium niobate. By using a different (shorter)grating period, the frequency conversion device can be used for sumfrequency generation. For example, a 1480 nm laser and a 920 nm lasercan be mixed to create 567 nm yellow light for medical applications.

An alternative configuration for generating infra-red light is theoptical parametric oscillator (OPO), where a single pump beam is usedand one or both of the signal and idler wavelengths are resonated in acavity formed by mirrors placed around the frequency converter as iswell known in the art. Once above threshold, (i.e. at high enough pumplevels to cause oscillation) the OPO is very efficient in transferringenergy from the pump to the signal and idler beams.

Although the present invention has been described in detail withreference to magnesium oxide doped congruent lithium niobate, MgO:CLN,it is equally applicable to other nonlinear optical materials known inthe art including: undoped congruent lithium niobate and tantalate,stoichiometric lithium niobate and tantalate materials, either grownfrom the melt or prepared by vapor transport equilibrium, magnesiumdoped stoichiometric lithium niobate (SLN) and tantalate (STN), Ti dopedand Ti diffused CLN, SLN and SLT and similar materials with otherdopants designed to reduce the photorefractive effect such as zinc dopedcongruent lithium niobate.

The detailed discussion of the present invention has been presented withreference to a substrate thickness of 0.5 mm since this is a standard,commercially available substrate dimension. The present invention isequally applicable to both thinner and thicker substrates, in particular0.25 mm and 1.0 mm substrates. Thicker substrates, such as 2 mm and 3mm, can be readily poled using this technique for high powerapplications especially in infra red generating devices. There is nolimitation on the wafer diameter or transverse dimensions of thesubstrate. If the illumination system cannot uniformly illuminate theentire substrate at once, the surface of the substrate may simply bemasked off with an opaque material, such as dicing tape, and the domaininversion process performed in sections across the substrate.

In one embodiment, the present invention relates to the fabrication ofdomain inverted structures. Generally the applications for these domaininverted devices are in the field of optical frequency conversion usingquasi-phase matching to provide efficient energy transfer from onewavelength to another. Whilst the present invention has been describedin detail with reference to domain inversion gratings, it should beunderstood that quasi-phasematched devices may contain periodic,aperiodic and pseudo-random phase reversal structures as required toproduce the desired phase matching curve. In addition, whilst a gratinggenerally consists of a multiple number of features arrayed periodicallyor aperiodically, in a domain inverted device it may consist of as fewas two domains, requiring only a single domain boundary Domain inversiondevices may also be used for applications other than optical frequencyconversion, such as polarization rotation, optical switching and opticalbeam deflection.

Throughout the embodiments described above, reference has been made tothe front and back surfaces of the substrate. These faces are not fixedwith respect to the crystal orientation and depend on thephotolithographically applied mask layers and the direction of theillumination applied to the crystal. In general, thephotolithographically masked face is referred to as the front face, andthe spatially uniform illumination is incident on the back face.

No limitations have been set on the period of the domain inversiongrating that can be realized with the techniques of the presentinvention. Isolated domains with submicron dimensions have beenobserved, offering the prospect of domain inversion gratings withperiods of less than 2 μm for quasi-phasematching of UV interactions.The visible frequency conversion applications which are the mostpromising application of the present invention require periods rangingfrom ˜4 μm for the blue, through ˜7 um for the green up to ˜12 μm forthe red. There is also no upper limit to the period which can befabricated, in fact isolated domains can be reliably fabricated usingthe present invention with high repeatability and precise domain sizecontrol for use in applications such as beam deflectors or optical totalinternal switches.

Although both the device fabrication and applications have been writtenwith reference to bulk frequency conversion applications, the frequencyconversion device fabricated by the present invention can also be usedas a substrate for highly efficient waveguide frequency conversionapplications. In this case, the tapered domain structure does not ensurethat there is a 50/50 duty cycle within the waveguide region, but bycareful control of the poling parameters the grating duty cycle at oneof the two crystal faces can be controlled to be substantially 50/50 for1^(st) order quasi-phasematching. Optical waveguides can be fabricatedin the MgO:CLN substrate using any of the techniques known in the art,such as annealed proton exchange (APE), reverse proton exchange (RPE),titainium indiffusion and zinc indiffusion. In the fabrication sequencefor APE and RPE devices the waveguide and periodic poling steps can beperformed in any order since neither substantially affects thecapability to perform the other. With the metal indiffusion waveguidesthe waveguide process is preferably performed first so that it does notdisturb the short period domain inversion during the high temperatureprocess. In this instance the present invention is particularlyimportant since the mobile charge electrode enables high quality domaininversion to be generated even through the metal indiffused waveguideregions. In general the same fabrication techniques are applied towaveguide frequency converters as for the bulk embodiments describedabove. The design approach for the devices is very similar, the opticalwaveguide mode effective index is used to compute the required gratingperiod for a given wavelength rather than the bulk crystal refractiveindex.

The embodiments described above serve the purpose of demonstrating theprinciple of the current invention. A person with ordinary skill-in-theart can derive more specific embodiments beyond those described herethat are in the spirit of the current invention. Techniques described inthe different embodiments can be freely combined to produce furtherembodiments which enhance the control of the domain growth.

1. A domain grating device, comprising: a substrate with first andsecond opposing surfaces, the substrate having an inverted domaingrating structure comprising a plurality of inverted domains that extendthrough the entire substrate, wherein an inverted domain average dutycycle at the first surface is greater than 50% and less than 100% and aninverted domain average duty cycle at the second surface is less than50% and greater than 0%, and wherein a length of an inverted domain atthe second surface is less than a length of the inverted domain at thefirst surface.
 2. The device of claim 1, wherein the length of theinverted domain at the second surface is no more than approximately 0.5microns less than the length of the inverted domain at the firstsurface.
 3. The device of claim 1, wherein the substrate is aferroelectric substrate.
 4. The device of claim 1, wherein the substrateis made of a material selected from at least one of, MgO doped congruentlithium niobate, stoichiometric Lithium Niobate, Stoichiometric LithiumTantalate, MgO:Stoichiometric Lithium Niobate, MgO:StoichiometricLithium Tantalate, ZnO:Lithium Niobate, In:Lithium Niobate, Ti:LithiumNiobate and Er:Lithium Niobate.
 5. The device of claim 1, wherein athickness of the substrate is between 100 μm and 2 mm.
 6. The device ofclaim 1 wherein a thickness of the substrate is between 450 μm and 550μm.
 7. The device of claim 1 wherein a thickness of the substrate isbetween 950 μm and 1150 μm.
 8. The device of claim 1, wherein athickness of the substrate is at least 2 mm.
 9. The device of claim 1,wherein said grating structure has an average period between 4 μm and 13μm.
 10. The device of claim 1, wherein said grating structure has anaverage period of less than 4 μm.
 11. The device of claim 1, whereinsaid grating structure has an average period that is greater than 15 μmand the substrate has a thickness greater than 1 mm.
 12. The device ofclaim 1, wherein a width of the inverted domain at the second surfacetapers from one end of the inverted domain to another end of theinverted domain.
 13. The device of claim 12, wherein the width tapers byno more than approximately 0.5 microns.
 14. The device of claim 1,wherein a domain pattern on each of the first and second surfaces is avisible domain pattern.
 15. The device of claim 14, wherein the visibledomain pattern is used to predict a performance of the frequencyconverter for process yield purposes.
 16. The device of claim 1, whereinthe surface at which the inverted domains have a greater duty cycle isthe surface where a pattern electrode is disposed during a polingprocess.
 17. The device of claim 1, further comprising: a repeatedinverted domain grating structure extending through the entire substrateforming a frequency conversion device; a laser pump source; and a meansconfigured to provide phasematching between said laser pump source andsaid frequency conversion device.
 18. The device of claim 17, where saidmeans configured to provide phasematching between said laser pump sourceand said frequency conversion device is a heating device configured tomaintain a particular temperature of said frequency conversion device.19. The device of claim 17, where said means configured to providephasematching between said laser pump source and said frequencyconversion device is a means to tune the wavelength of the pump sourceto match that of the frequency converter.
 20. The device of claim 17,where said laser pump source comprises a semiconductor diode laser. 21.The device of claim 17, where said laser pump source comprises a diodepumped solid state laser.
 22. The device of claim 17, where the opticalbeam emitted by said pump laser source passes through the region of thedomain grating which has substantially 50% duty cycle.
 23. The device ofclaim 17, further comprising a means for efficient optical couplingbetween said laser pump source and said frequency conversion device. 24.The device of claim 17, wherein the inverted domain device isincorporated inside the laser cavity of said pump laser.
 25. The deviceof claim 17, wherein the inverted domain device is at least partiallycoated with a conductive coating forming a charge dissipatingclosed-loop.
 26. The device of claim 17, wherein said pump laser sourcecomprises a plurality of individual laser beams.
 27. The device of claim17, wherein the substrate is made of a material selected from at leastone of, MgO doped congruent lithium niobate, stoichiometric LithiumNiobate, Stoichiometric Lithium Tantalate, MgO:Stoichiometric LithiumNiobate, MgO:Stoichiometric Lithium Tantalate, ZnO:Lithium Niobate,In:Lithium Niobate, Ti:Lithium Niobate and Er:Lithium Niobate.
 28. Thedevice of claim 17, wherein a thickness of the substrate is between 100μm and 2 mm.
 29. The device of claim 17, wherein a period of the gratingstructure is between 4 μm and 7 μm.
 30. The device of claim 17, whereinthe length of the inverted domain at the second surface is no more thanapproximately 0.5 microns less than the length of the inverted domain atthe first surface.
 31. The device of claim 30, wherein a width of theinverted domain at the second surface tapers from one end of theinverted domain to another end of the inverted domain.
 32. The device ofclaim 31, wherein the width tapers by no more than approximately 0.5microns.
 33. The device of claim 17, further comprising: an opticalcoupling means; a spatial light modulator; a projector lens element; anda screen.
 34. The device of claim 33, wherein the substrate is made of amaterial selected from at least one of, MgO doped congruent lithiumniobate, stoichiometric Lithium Niobate, Stoichiometric LithiumTantalate, MgO:Stoichiometric Lithium Niobate, MgO:StiochiometricLithium Tantalate, ZnO:Lithium Niobate, In:Lithium Niobate, Ti:LithiumNiobate and Er:Lithium Niobate.
 35. The device of claim 33, wherein athickness of the substrate is between 100 μm and 2 mm.
 36. The device ofclaim 33, wherein the grating structure has an average period between4.0 μm and 13 μm.
 37. The device of claim 36, wherein inverted domainsare tapered in size from the first surface to the second surface.
 38. Amethod of creating a domain grating device in a substrate comprising:providing electrical contacts to first and second opposing surfaces ofsaid substrate; generating mobile charges in said substrate; applyingpotentials to said electrical contacts creating a patterned current flowthrough said substrate; and forming an inverted domain grating structurehaving an inverted domain average duty cycle at the first surface thatis greater than 50% and less than 100% and an inverted domain averageduty cycle at the second surface that is less than 50% and greater than0%, wherein a length of an inverted domain at the second surface is lessthan a length of the inverted domain at the first surface.
 39. Themethod of claim 38, wherein the length of the inverted domain at thesecond surface is no more than approximately 0.5 microns less than thelength of the inverted domain at the first surface.
 40. The method ofclaim 38, wherein a width of each inverted domain at the second surfacetapers from one end of the inverted domain to another end of theinverted domain by no more than approximately 0.5 microns.
 41. Themethod of claim 40, wherein the width tapers by no more thanapproximately 0.5 microns.
 42. The method of claim 38, wherein a domainduty cycle at the first surface is greater than 50% and less than 100%,and a domain duty cycle at the second surface is 0%.
 43. The method ofclaim 38, wherein mobile charges are generated in the substrate incombination with the application of a patterned electric field.
 44. Themethod of claim 38, further comprising: annealing the domain gratingdevice at a temperature between 500 and 650 degrees centigrade for atime of between 24 and 72 hours; and providing a closed-loop electricaldischarge path between the opposing faces of the domain grating deviceduring said annealing.
 45. The method of claim 38, further comprising:generating said mobile charges by exposing the substrate tosubstantially spatially uniform optical illumination through at leastone of the first and second surfaces to form an illuminated face. 46.The method of claim 45, wherein the optical illumination consists ofwavelengths from 250 nm to 600 nm.
 47. The method of claim 45, whereinthe optical illumination consists of wavelengths greater than or equalto 400 nm.
 48. The method of claim 45, wherein the optical illuminationis filtered to remove wavelengths shorter than 320 nm.
 49. The method ofclaim 45, further comprising: disposing a uniform transparent electrodeon the illuminated face.
 50. The method of claim 49, further comprising:disposing a patterned electrode on an opposite face relative to theilluminated face.
 51. The method of claim 50, further comprising:applying a high voltage to the electrical contacts to cause aphotocurrent to flow through the substrate; and controlling a magnitudeand duration of the high voltage and illumination to create a domaininversion structure through the thickness of the substrate.
 52. Themethod of claim 51, further comprising: applying illumination to thesubstrate simultaneously with application of a first voltage.
 53. Themethod of claim 52, further comprising: terminating the application ofillumination in response to a parameter selected from at least one of,time, current flow and charge transfer.
 54. The method of claim 53,further comprising: applying a second voltage to the substrate crystalafter the illumination is terminated to cause a poling current to flow.55. The method of claim 54, further comprising: terminating the secondvoltage in response to a parameter selected from at least one of, time,current flow and charge transfer to create a domain inversion structure.56. The method of claim 55, further comprising: applying a time delaybetween termination of the illumination and application of the secondvoltage.